Geology topics
D.M. Hopkins
Publications and source records attributed to D.M. Hopkins.
Locality map for scintillometer and analytical data collected in 1981, Ugashik and Karluk quadrangles, Alaska
Scintillometer data were collected routinely in 1981 as part of the field investigations in the Ugashik and Karluk quadrangles. The instrument used was a Geometrics Model GR-101A, mounted in the helicopter. The data, in counts per second (cps), were obtained at most of the sites by the helicopter pilot (Svec) while geologists were occupying the sample site. Scintillometer localities are indicated on the map by a dot. The sites are concentrated in the southeastern part of the map area where previous work (OF 81-253) had outlined an area with anomalously high cps. The bedrock at these sites is a Late Tertiary hornblende granodiorite instrusive dated at 9.4 my. Hornfelsed sedimentary rock overlying the batholith gave cps nearly as high as the intrusive rock. Samples routinely checked by semiquantitative spectrographic analysis for thorium failed to detect the element. In general the Ugashik and Karluk quadrangles have a low level of gamma radiation. A majority of the sites, 52 percent, gave readings of less than 35 cps, and only 10 percent had values of greater than cps. Thick alluvium or glacial deposits generally gave low values. Readings from sedimentary bedrock were extremely erratic and no generalization can be made concerning their level of gamma radiation. The hornblende granodiorite and overlying hornfelsed sedimentary rock are all greater than 30 cps, but again, the values are somewhat erratic and no definitive pattern is readily apparent. Bedrock samples are collected for analysis are on the map by a triangle.
Geochemical analyses of stream sediments and heavy-mineral concentrates collected near a stratabound Cu-Ag occurrence in the Cabinet Mountains Wilderness, Montana
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Arctic oceanic climate in late Cenozoic time
Faunal and lithologic evidence is used to reconstruct paleoceanographic events over the last 4.5 million years. The inception of perennial sea-ice cover is dated at about 0.7 million years.
Location map and summary logs for the U.S. Geological Survey's 1979 Beaufort Sea over the ice drilling program
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Preliminary report on the petrography and geochemistry of basalt from the Pribilof Islands and vicinity, southern Bering Sea
Alkali basalt from the Pribilof Islands has been described by several workers (Washington and Keyes, 1930; Barth, 1956; Cox and others, 1966; and Kay, 1977). This report complements the previous reports by providing new petrographic and geochemical data for samples from St. Paul and St. George Islands and from two dredge hauls near St. George Island (Fig. 1). The geologic setting of the Pribilof Islands region is given by Barth (1956), Hopkins and Einarsson (1965), Cox and others (1966), Hopkins and others (1969), Marlow and others (1976), and Hopkins and Silberman (1978). Quaternary basalt (Table 1) (Cox and others, 1966; Hopkins, 1976; Simpson and others, 1979) is extruded onto the structurally high Pribilof Ridge (Marlow and others, 1976) which consists of rocks as old as late Jurassic (Vallier and others, 1979). Barth (1956) recognized the structural implication of alkalic volcanism in back-arc environments. We plan to interpret alkalic volcanism in the Pribilof Islands within the context of recent tectonic and petrologic findings in the southern Bering Sea. Funds for this project were provided jointly by the U.S. Geological Survey and Bureau of Land Management. Funds from the Bureau of Land Management were managed by the Outer Continental Shelf Environmental Assessment Program (OCSEAP) office of the National Oceanic and Atmospheric Administration (NOAA).
Potassium-argon ages of basement rocks from Saint George Island, Alaska
Potassium-argon ages from basement granitic rocks that intrude serpentinized peridotite on St. George Island, Alaska, range from 50 to 57 million years, with an initial argon isochron age. interpreted as a minimum figure, of 52 ± 2 m.y. The age of the granitic rocks and their association with serpentinized peridotite near the continental margin, which apparently marks the site of a Mesozoic subduction zone, suggest that the position of the subduction zone shifted southward to the Aleutian Trench in early Tertiary time.
Geological and geochemical investigations of uranium occurrences in the Arrastre Lake area of the Medicine Bow Mountains, Wyoming
Metasedimentary rocks of Precambrian X age in and near the Snowy Range wilderness study area of southeastern Wyoming are lithologically and chronologically similar to those on the north shore of Lake Huron in Canada. The rocks in Canada contain major deposits of uranium in quartz-pebble conglomerates near the base of the metasedimentary sequence. Similar conglomerates in the Deep Lake Formation in the Medicine Bow Mountains of southeastern Wyoming are slightly radioactive and may contain deposits of uranium and other valuable heavy metals. During the summer of 1976, a geological and geochemical pilot study was conducted in the vicinity of Arrastre Lake in the Medicine Bow Mountains to determine the most effective exploration methods for evaluating the uranium potential of the Snowy Range wilderness study area. The area around Arrastre Lake was selected because of the presence of a radioactive lens within a quartz-pebble conglomerate of the Deep Lake Formation. The results of the survey indicate possible uranium mineralization in the subsurface rocks of this formation. The radon content of the dilute waters of the area is much higher than can be accounted for by the uranium content of the surface rocks. Two sources for the high content of the radon are possible. In either case, the high values of radon obtained in this study are a positive indication of uranium mineralization in the subsurface rocks. The determination of the radon content of water samples is the recommended geochemical technique for uranium exploration in the area. The determination of uranium in water and in organic-rich bog material is also recommended.
Distribution of bottom sediments on the continental shelf, northern Bering Sea
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An improved ion-selective electrode method for the rapid determination of fluorine in rocks and soil
An improved method based on an ion-selective electrode technique for the analysis of fluorine in rocks and soils is presented. Analyses are made by using a sodium carbonate-potassium carbonate fusion and a citric acid dissolution of the fuseate. Prior to determining the fluorine concentration by a standard-addition procedure, sodium citrate buffer is added to the solution. The proposed method yields fluorine values in agreement with known values for standard rocks. Values of this method are also presented for six geochemical exploration rock-and-soil reference samples and compared to those from other ion-selective electrode techniques. Fifty samples can be analyzed in 1 person-day. The sensitivity of the method is 100 parts per million, and samples containing up to 45 percent fluorine have been successfully analyzed.
Physiographic subdivisions of the Chirikov Basin, northern Bering Sea
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Comments on the use of Hiatella arctica for determining cenozoic sea temperatures
[No abstract available]
A Pliocene flora and insect fauna from the Bering Strait region
A flood-plain forest has been preserved beneath a lava flow that invaded the Inmachuk River Valley in the northern part of the Seward Peninsula, Alaska, during the Pliocene Epoch. The fossil flora is of great biogeographic interest because of its position (Fig. 1) in a tundra region about 250 km east of Bering Strait, 75 km south of the Arctic Circle, and 65 km west of the northwestern limit of spruce-birch forest. It provides insight into the history of the development of the circumpolar boreal forest (taiga). A rich arthropod fauna casts light on the phylogeny of several modern insect genera and on the origin of modern tundra faunas. A potassium-argon analysis of the overlying basaltic lava provides our first radiometric age estimate (5.7±0.2 million years) for the Clamgulchian Stage, a Late Tertiary time-stratigraphic unit based on fossil plants and widely recognized in Alaska ( Wolfe and Hopkins 1967) and northeastern Siberia.
The structure and origin of the large submarine canyons of the Bering Sea
Three exceptionally large and long submarine canyons — Bering, Pribilof, and Zhemchug — incise the continental slope underlying the southeastern Bering Sea. Bering Canyon, the world's longest known slope valley, is approximately 400 km long and has a volume of 4,300 km 3 . The volume of Pribilof Canyon is 1,300 km 3 and that of Zhemchug is 8,500 km 3 ; Zhemchug Canyon may well be the world's largest slope valley; most other large submarine canyons have volumes less than 500 km 3 . Pribilof and Zhemchug canyons are further distinguished by the headward bifurcation of their slope axes to form elongated trough-shaped basins behind the regionally projected position of the shelf edge. These troughs are superimposed over structural depressions formed by down-faulted basement rocks of Mesozoic and older ages. Prior to canyon cutting these depressions were filled with as much as 2,600 m of shallow-water diatomaceous, tuffaceous, and detrital sediments largely of Tertiary age. Deposition of these sediments took place concurrently with general margin subsidence of at least 2,000 m. The data and conclusions presented in this paper stress that the location, trend, and shape of the enormous submarine canyons cutting the Bering margin are structurally determined. However, axial cutting and headward erosion within the relatively unconsolidated Tertiary strata and the older, lithified basement rock is thought to have been caused by basinward-sliding masses of sediment; these unstable sediment bodies accumulated on the upper continental slope and outer shelf, probably near the mouths of major Alaskan rivers. Bering Canyon was periodically cut and filled by axial sedimentation during Late Tertiary and Quaternary time. Pribilof and Zhemchug canyons, however, are thought to have been excavated entirely during the Pleistocene. It is presumed that, during one or more periods of glacially lowered sea level, the Kuskokwim and Yukon rivers emptied into or near the heads of Pribilof and Zhemchug canyons. The enormous size and unusual shape of Zhemchug Canyon resulted from the breaching of the seaward wall of an outer-shelf basement depression and the subsequent removal of nearly 4,500 km 3 of Tertiary deposits filling it.
Geologic history of the continental margin of North America in the Bering Sea
The North American continental margin beneath the Bering Sea is nearly 1,300 km long and extends from Alaska to eastern Siberia. The margin is a canyon-scarred 3,200–3,400-m high escarpment separating one of the world's largest epicontinental seas (the shallow Bering Sea) and the Aleutian Basin (the deep-water Bering Sea), a marginal oceanic basin distinguished by having its southern boundary formed by the Aleutian Ridge. Three geomorphic provinces can be recognized: a southeastern province characterized by a gentle continental slope (lacking V-shaped canyons) and an outlying continental borderland (formed by Umnak Plateau); a central province distinguished by a steep canyon-scarred slope, and a northwestern province having a gentler and, apparently, less eroded continental slope. Continuous seismic reflection profiles show that the margin is constructed of three major structural-stratigraphic units: ( 1 ) an acoustic basement underlying the outer shelf and upper slope; ( 2 ) an overlying main layered sequence; and ( 3 ) a stratified rise unit underlying and forming the continental rise at the base of the slope. The existing margin evolved with downbowing and faulting of the acoustic basement, an older margin probably of Late Mesozoic age, consisting in part of well-indurated siltstone and mudstone, in Early Tertiary time. Concomitant with subsidence as much as 1,500 m of main-layered-sequence strata were draped over the basement. Intense canyon cutting, presumed to have been caused by the rapid deposition of unstable masses of riverborn sediment over the outer shelf and upper slope, is thought to have begun in Late Tertiary and Quaternary time. Concurrent with canyon cutting, submarine fans, consisting of turbidites forming the rise unit, accrued at the base of the continental slope. Subsidence of the continental margin during the Tertiary may be related to foundering (“oceanization”) of a continental block to form the Aleutian Basin, or to simple isostatic depression of a former segment of the North Pacific oceanic floor in response to sediment infilling north of the Aleutian Ridge.