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NARROW
GeoRef Subject
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all geography including DSDP/ODP Sites and Legs
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Bear Lake (2)
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Cascade Range (2)
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North America
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elements, isotopes
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metals
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sulfides
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Primary terms
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carbon
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Oakville Sandstone (2)
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North America
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Paleozoic
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Bear Lake is a long-lived lake filling a tectonic depression between the Bear River Range to the west and the Bear River Plateau to the east, and straddling the border between Utah and Idaho. Mineralogy, elemental geochemistry, and magnetic properties provide information about variations in provenance of allogenic lithic material in last-glacial-age, quartz-rich sediment in Bear Lake. Grain-size data from the silici-clastic fraction of late-glacial to Holocene carbonate-rich sediments provide information about variations in lake level. For the quartz-rich lower unit, which was deposited while the Bear River flowed into and out of the lake, four source areas are recognized on the basis of modern fluvial samples with contrasting properties that reflect differences in bedrock geology and in magnetite content from dust. One of these areas is underlain by hematite-rich Uinta Mountain Group rocks in the headwaters of the Bear River. Although Uinta Mountain Group rocks make up a small fraction of the catchment, hematite-rich material from this area is an important component of the lower unit. This material is interpreted to be glacial flour. Variations in the input of glacial flour are interpreted as having caused quasi-cyclical variations in mineralogical and elemental concentrations, and in magnetic properties within the lower unit. The carbonate-rich younger unit was deposited under conditions similar to those of the modern lake, with the Bear River largely bypassing the lake. For two cores taken in more than 30 m of water, median grain sizes in this unit range from ~6 μm to more than 30 μm, with the coarsest grain sizes associated with beach or shallow-water deposits. Similar grain-size variations are observed as a function of water depth in the modern lake and provide the basis for interpreting the core grain-size data in terms of lake level.
Paleomagnetism and environmental magnetism of GLAD800 sediment cores from Bear Lake, Utah and Idaho
A ~220,000-year record recovered in a 120-m-long sediment core from Bear Lake, Utah and Idaho, provides an opportunity to reconstruct climate change in the Great Basin and compare it with global climate records. Paleomagnetic data exhibit a geomagnetic feature that possibly occurred during the Laschamp excursion (ca. 40 ka). Although the feature does not exhibit excursional behavior (≥40° departure from the expected value), it might provide an additional age constraint for the sequence. Temporal changes in salinity, which are likely related to changes in freshwater input (mainly through the Bear River) or evaporation, are indicated by variations in mineral magnetic properties. These changes are represented by intervals with preserved detrital Fe-oxide minerals and with varying degrees of diagenetic alteration, including sulfidization. On the basis of these changes, the Bear Lake sequence is divided into seven mineral magnetic zones. The differing magnetic mineralogies among these zones reflect changes in deposition, preservation, and formation of magnetic phases related to factors such as lake level, river input, and water chemistry. The occurrence of greigite and pyrite in the lake sediments corresponds to periods of higher salinity. Pyrite is most abundant in intervals of highest salinity, suggesting that the extent of sulfidization is limited by the availability of SO 4 2‒ . During MIS 2 (zone II), Bear Lake transgressed to capture the Bear River, resulting in deposition of glacially derived hematite-rich detritus from the Uinta Mountains. Millennial-scale variations in the hematite content of Bear Lake sediments during the last glacial maximum (zone II) resemble Dansgaard-Oeschger (D-O) oscillations and Heinrich events (within dating uncertainties), suggesting that the influence of millennial-scale climate oscillations can extend beyond the North Atlantic and influence climate of the Great Basin. The magnetic mineralogy of zones IV–VII (MIS 5, 6, and 7) indicates varying degrees of post-depositional alteration between cold and warm substages, with greigite forming in fresher conditions and pyrite in the more saline conditions.
Late Quaternary eolian and alluvial response to paleoclimate, Canyonlands, southeastern Utah
Magnetic Ilmenite-Hematite Detritus in Mesozoic-Tertiary Placer and Sandstone-Hosted Uranium Deposits of the Rocky Mountains
Post-Mazama (7 KA) Faulting Beneath Upper Klamath Lake, Oregon
Abstract As part of tectonic studies by the Energy Program of the U.S. Geological Survey, we have modeled aeromagnetic anomalies over the coastal plain of the Arctic National Wildlife Refuge (ANWR), Alaska. Preliminary models indicate that the lineated, moderate-intensity anomalies produced by shallow sources within the coastal plain are best fit by a series of stratigraphic layers with both normal and reversed remanent magnetization. The layers follow seismically determined stratigraphic and structural boundaries from near the surface to depths of 1 to 2 km. The modeled total magnetic intensities range up to .115 A/m for the reversely magnetized units and up to .069 A/m for the normally magnetized units. Based on these models, we suspect that the magnetic anomalies are primarily the result of detrital remanent magnetization that formed as the sediments were deposited. Another plausible explanation involves chemical remanence, acquired rapidly with respect to geomagnetic polarity reversals, as the marine turbidite sediments accumulated, thus producing a stratigraphically ordered polarity sequence. The high total magnetizations and reversed polarities leave open the additional possibility that thick sequences of originally reversed magnetization were overprinted by normal remanence through some stratigraphically controlled mechanism.
Record of middle Pleistocene climate change from Buck Lake, Cascade Range, southern Oregon—Evidence from sediment magnetism, trace-element geochemistry, and pollen
Late Cretaceous remagnetization of Proterozoic mafic dikes, southern Highland Mountains, southwestern Montana: A paleomagnetic and 40 Ar/ 39 Ar study
Abstract Post-depositional iron-sulfide (Fe-S) minerals that are related to hydrocarbon seepage have changed the original magnetizations at Cement oil field (Anadarko basin, Oklahoma), at Simpson oil field (North Slope basin, Alaska), and above deep Cretaceous oil and gas reservoirs, south Texas coastal plain. At Cement, ferrimagnetic pyrrhotite (Fe 7 S 8 ) formed with pyrite and marcasite in Permian red beds. The Fe-S minerals contain sulfur from two sources: (1) abiogenic sulfide, which has positive δ 34 S values, derived from thermochemical reduction of sulfate in deep reservoirs; and (2) biogenic sulfide, which has negative δ 34 S values, produced by reactions mediated by sulfate-reducing bacteria fed by leaking hydrocarbons. At Simpson, ferrimagnetic greigite (Fe 3 S 4 ) dominates magnetizations in nonmarine Upper Cretaceous clastic beds that contain epigenetic sulfide (δ 34 S > +20 per mil) and seeping biodegraded oil. In this setting, the authigenic magnetic sulfide mineral apparently incorporated sulfide produced by bacterial sulfate reduction under limited sulfate conditions. An inferred hydrocarbon food source for the sulfate-reducing bacteria links the hydrocarbon seepage to the greigite. The greigite is perhaps forming today. In middle Tertiary sandstones of southeast Texas, pyrite and marcasite formed when abiogenic H 2 S (enriched in 34 S) migrated upward from deep reservoirs, or when H 2 S (depleted in 34 S) was produced at shallow depths by bacteria that used organic material dissolved in migrating water from depth. The pyrite and marcasite replaced detrital magnetic iron-titanium oxide minerals. The degree of such replacement appears to increase toward faults that connect deep petroleum reservoirs to shallow sandstone. Our results show that abiologic and biologic mechanisms can generate different magnetic sulfide minerals in some sulfidic zones of hydrocarbon seepage. More commonly the magnetizations in such zones would be diminished as a result of the replacement of detrital magnetic minerals by the common nonmagnetic sulfide minerals, or would remain unchanged if such detrital minerals were originally absent.
High-Resolution Aeromagnetic Study of the New Madrid Seismic Zone: A Preliminary Report
Genesis of the tabular-type vanadium-uranium deposits of the Henry Basin, Utah; reply
Genesis of the tabular-type vanadium-uranium deposits of the Henry Basin, Utah
Iron sulfide minerals at Cement oil field, Oklahoma: Implications for magnetic detection of oil fields
Source of Anomalous Magnetization in Area of Hydrocarbon Potential: Petrologic Evidence from Jurassic Preuss Sandstone, Wyoming-Idaho Thrust Belt
Abstract The Mariano Lake uranium deposit, hosted by the Brushy Basin Member of the Jurassic Morrison Formation, occurs in the Smith Lake district of the Grants uranium region, New Mexico. The orebody, contains abundant amorphous organic material, which suggests that it represents a primary-type deposit; however, the orebody is close to a regional reduction-oxidation interface, which suggests that uranium was secondarily redistributed by oxidative processes. Uranium contents correlate positively with organic carbon contents. Petrographic evidence points to uranium residence in amorphous organic material that was post- depositionally introduced in the diagenetic history of the host sandstone. Uranium mineralization was preceded by precipitation of pyrite (δ 34 S values of — 11.0 to — 38.2 per mil), mixed-layer smectite-illite clays, and quartz and potassium feldspar overgrowths; and also partial dissolution of some detrital feldspars. Alterations associated with uranium mineralization include precipitation of the organic material, microcrys- talline quartz, and pyrite and marcasite (δ 34 S values of -29.4 to -41.6 per mil), and the destruction of detrital Fe-Ti oxide grains. Following mineralization, calcite, dolomite, barite, and kaolinite were precipitated, and some iron disulfides were replaced by ferric oxides. Geochemical data and petrographic observations both indicate that the Mariano Lake orebody is a primary-type deposit. Oxidative processes have not noticeably redistributed uranium in the immediate vicinity of the deposit, nor have they greatly modified geochemical characteristics in the ore. Impedance of ground-water flow by local folds and the lower porosity characteristics of ore zones may have helped to preserve the deposit.
Abstract Petrographic study of the Mariano Lake-Lake Valley cores reveals three distinct zones of postdepositional alteration of detrital Fe-Ti (iron-titanium) oxide minerals in the Westwater Canyon Member of the Upper Jurassic Morrison Formation. In the uranium-bearing and adjacent portions of the Westwater Canyon, these detrital Fe-Ti oxide minerals have been thoroughly altered by leaching of iron. Stratigraphically lower parts of the Westwater Canyon and the underlying Recapture Member are characterized by preservation of Fe-Ti oxide grains, primarily magnetite and ilmenite, and of hematite, and by an absence of uranium concentrations. Partly destroyed Fe-Ti oxide minerals occupy an interval between the zones of destruction and preservation. Alteration patterns of the Fe-Ti oxide minerals are reflected in bore-hole magnetic susceptibility logs. Magnetic susceptibility response in the upper parts of the Westwater Canyon Member is flat and uniformly < 500 /xSI units, but at greater depths it fluctuates sharply, from <1,000 to nearly 8,000 μSI units. The boundary between uniformly low and high magnetic susceptibility response corresponds closely to the interval that divides the zone of completely altered from the zone of preserved detrital Fe-Ti oxide minerals. The alteration pattern suggests that solutions responsible for destruction of the Fe-Ti oxide minerals originated in the overlying Brushy Basin Member of the Morrison Formation. Previous studies indicate that these solutions were rich in soluble organic matter and perhaps in uranium. Uranium precipitation may have been controlled by a vertically fluctuating interface between organic-rich solutions and geochemically different fluids in which the detrital Fe-Ti oxide minerals were preserved.