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R. L. Reynolds

Publications and source records attributed to R. L. Reynolds.

30 records · Page 2Linked to original sources
USGS1996

Environmental magnetic implications of greigite (Fe3S4) formation in a 3 m.y. lake sediment record from Butte Valley, northern California Andrew

Authigenic greigite (Fe 3 S 4 ) has been identified in several horizons of lake beds in a 102-m core from Butte Valley, northern California, using mineral magnetic methods and x-ray diffraction analysis. The presence of greigite has several implications for the paleoenvironmental record from Butte Valley. First, its occurrence in 2.5–3.0 Ma strata confirms that greigite can persist in the geological record for long periods of time. Second, the detrital mineral magnetic record may be partially obscured by the presence of authigenic greigite and care must be taken in interpreting magnetic variations in the greigite-bearing zones as paleoclimate proxies. Third, differences in the timing of remanence acquisition for authigenic and detrital phases may compromise studies of high-frequency geomagnetic field variations. Fourth, greigite may also be significant as a paleoenvironmental indicator of lake and sediment chemistry. The magnetic detection of greigite may therefore provide important information about paleolimnological conditions.

Geophysical Research Letters↗
USGS1996

Late Cretaceous remagnetization of Proterozoic mafic dikes, southern Highland Mountains, southwestern Montana: A paleomagnetic and 40Ar/39Ar study

Paleomagnetic results from Early Proterozoic metabasite sills and Middle Proterozoic diabase dikes from the southern Highland Mountains of southwestern Montana give well-defined, dual-polarity magnetizations that are statistically identical to those from a small Late Cretaceous pluton that cuts the dikes. The concordance of paleomagnetic directions from rocks of three widely separated ages indicates that the Proterozoic rocks were remagnetized, probably during Late Cretaceous time. Paleomagnetic, rock magnetic, and petrographic observations from the metabasite and diabase samples indicate that remanence is carried primarily by low-Ti magnetite. Combining virtual geomagnetic poles from metabasite sills, diabase dikes, and the Late Cretaceous pluton, we obtain a paleomagnetic pole at 85.5°N, 310.7°E ( K = 19.9, A 95 = 9.1°, N = 14 sites) that is similar to a reference pole from the 74 Ma Adel Mountain Volcanics of western Montana. Biotite and hornblende 40 Ar/ 39 Ar isotopic dates from host basement gneiss and a hornblende from a remagnetized metabasite sill yield ages of ca. 1800 Ma; these dates probably record cooling of the southern Highland Mountains following high-grade metamorphism at 1.9–1.8 Ga. The gneiss and metabasite age spectra show virtually no evidence of disturbance, indicating that the basement rocks were never heated to temperatures sufficient to cause even partial resetting of their argon systems. Thus, the overprint magnetization of the Highland Mountains rocks is not a thermoremanent magnetization acquired during conductive cooling of nearby Late Cretaceous plutons. Remagnetization of the metabasite sills and diabase dikes was probably caused by localized thermochemical and thermoviscous effects during circulation of Late Cretaceous hydrothermal fluids related to epithermal mineralization. The absence of significant disturbance to the 40 Ar/ 39 Ar age spectrum from the remagnetized metabasite hornblende indicates that some secondary magnetizations may go unrecognized and undated, even if 40 Ar/ 39 Ar dating is applied.

Montana↗
USGS1994

Sediment magnetic and paleomagnetic data from Buck Lake, Oregon

Sediment magnetic and paleomagnetic studies were conducted on a core from Buck Lake, Klamath County, Oregon, that was collected as part of an investigation into the Quaternary climate history of the western United States. This report documents the methods used to obtain paleomagnetic directions, magnetic properties, and ancillary data, and presents these data in tabular form. Adam (1993) and Adam and others (1994) describe the site, the drilling methods, and lithology of the lacustrine sediments. Rosenbaum and others (1994) present preliminary interpretations of the sediment magnetic data and show that variations in magnetic properties closely reflect changes in climate as interpreted from the pollen record.

Open-File Report↗
USGS1993

The analysis of forms of sulfur in ancient sediments and sedimentary rocks: comments and cautions

Assumptions commonly made during analysis of the amount of monosulfides [acid-volatile sulfides (AVS)] and disulfides in modern sediments, may not be valid for ancient sedimentary rocks. It is known that ferric iron can oxidize H2S during AVS analysis unless a reducing agent such as stannous chloride is added to the treatment. In addition, some monosulfides such as greigite and pyrrhotite require heat during the AVS analysis in order to dissolve completely. However, the use of heat and/or stannous chloride in the AVS treatment may partially dissolve disulfides and it is generally recommended that stannous chloride not be used in the AVS treatment for modern sediments. Most of the monosulfides are assumed to be recovered as AVS without the addition of stannous chloride. This study investigates the recovery of monosulfides during sulfur speciation analysis with application to ancient sedimentary rocks. Sulfur in samples containing naturally occurring greigite and mackinawite or pyrite was measured using variations of a common sulfur-speciation scheme. The sulfur-speciation scheme analyzes for monosulfide sulfur, disulfide sulfur, elemental sulfur, inorganic sulfate and organically bound sulfur. The effects of heat, stannous chloride and ferric iron on the amounts of acid-volatile sulfide and disulfide recovered during treatment for AVS were investigated. Isotopic compositions of the recovered sulfur species along with yields from an extended sulfur-speciation scheme were used to quantify the effects. Hot 6 N HCl AVS treatment recovers > 60% of the monosulfides as AVS in samples containing pure greigite and mackinawite. The remaining monosulfide sulfur is recovered in a subsequent elemental sulfur extraction. Hot 6 N HCl plus stannous chloride recovers 100% of the monosulfides as AVS. The addition of ferric iron to pure greigite and mackinawite samples during AVS treatment without stannous chloride decreased the amount of monosulfides recovered as AVS and, if present in great enough concentration, oxidized some of the AVS to a form not recovered in later treatments. The hot stannous chloride AVS treatments dissolve <5% of well-crystallized pyrite in this study. The amount of pyrite dissolved depends on grain size and crystallinity. Greigite in ancient sedimentary rocks was quantitatively recovered as AVS only with hot 6 N HCl plus stannous chloride. Hot 6 N HCl AVS treatment of these rocks did not detect any monosulfides in most samples. A subsequent elemental sulfur extraction did not completely recover the oxidized monosulfides. Therefore, the use of stannous chloride plus heat is recommended in the AVS treatment of ancient sedimentary rocks if monosulfides are present and of interest. All assumptions about the amount of monosulfides and disulfides recovered with the sulfur-speciation scheme used should be verified by extended sulfur-speciation and/or isotopic analysis of the species recovered. ?? 1993.

Chemical Geology↗
USGS1990

Magnetic forward models of Cement oil field, Oklahoma, based on rock magnetic, geochemical, and petrologic constraints

Magnetic forward models of the Cement oil field, Oklahoma, were generated to assess the possibility that ferrimagnetic pyrrhotite related to hydrocarbon seepage in the upper 1 km of Permian strata contributes to aeromagnetic anomalies at Cement. Six bodies having different magnetizations were constructed for the magnetic models, based on geology and on petrologic and geochemical results, supplemented by rock magnetic measurements of shallow-core and outcrop samples. The column of rock through which hydrocarbons have passed is divided into three sulfide zones on the basis of pyrrhotite content, and the column is capped by a 30 m thick zone that contains ferric oxide minerals formed mainly from oxidized pyrite. Red beds unaffected by sulfidization, as well as a zone of rock depleted in hematite but lacking sulfide, surround sulfidic zones.The synthetic magnetic profiles are controlled mainly by pyrrhotite-bearing strata at depths of 200-500 m. The magnetizations of these bodies are calculated from: (1) petrographic estimates of pyrrhotite content relative to pyrite; (2) content of sulfide sulfur determined from chemical analysis; and (3) values for the magnetic susceptibility of monoclinic pyrrhotite. Total magnetizations of the bodies of highest pyrrhotite content range from about 3 X 10 (super -3) to 56 X 10 (super -3) A/m in the present field direction and yield magnetic anomalies (at 120 m altitude) having amplitudes of less than 1 nT to approximately 6 to 7 nT, respectively. Such amplitudes are much lower than those (as high as 60 nT) reported from the original total-field survey over the Cement field.Numerous assumptions were made in the generation of the models, and thus the results neither prove nor disprove the existence of aeromagnetic anomalies related to hydrocarbon seepage at Cement. Nevertheless, the results suggest that pyrrhotite, formed via hydrocarbon reactions and within a range of concentrations estimated at Cement, is capable of causing magnetic anomalies.

Geophysics↗
USGS1985

Uranium mineralization in the Smith Lake district of the Grants uranium region, New Mexico.

The Mariano Lake and Ruby 1 uranium orebodies, which together comprise much of the uranium ore in the Smith Lake district of the Grants uranium region, New Mexico, occur in sandstones in the lower part of the Brushy Basin Member of the Upper Jurassic Morrison Formation. The orebodies, which are offset by faults of Laramide age, are enriched in an amorphous organic material that was introduced into the host sandstone after deposition. The enrichment by this organic material, in ore, is an important characteristic of the primary uranium deposits in the Grants uranium region. However, the close proximity of the chemically reduced ore zones to oxidized rock is suggestive that the deposits represent accumulations of uranium redistributed by reduction-oxidation processes from preexisting primary deposits.Within the ores, whole-rock abundances of organic carbon correlate positively with uranium contents. This correlation is consistent with petrologic evidence which indicates that uranium is everywhere intimately admixed with the amorphous organic material. Ore zones are also enriched in vanadium (as ore-stage vanadiferous chlorite) and sulfur (as ore-stage iron disulfide minerals with delta 34 S values ranging from -29 to -42ppm).Petrographic observations demonstrate that Smith Lake uranium mineralization occurred early (before major burial compaction) in the paragenetic sequence of host-rock diagenetic alterations but was preceded by precipitation of authigenic iron disulfides (delta 34 S values ranging from -11 to -38ppm), mixed-layered smectite-illite clays, and potassium feldspars.Additional preore alterations included dissolution of detrital sanidine and plagioclase and the leaching of iron from detrital iron-titanium oxide grains. Following mineralization, varying amounts of authigenic calcite and barite formed, both of which were partly replaced by kaolinitc. Oxidation of some previously formed iron disulfide minerals occurred late in the paragenetic sequence, as did localized precipitation of native selenium, pyrite, and very minor amounts of uranium minerals.The positive correlation between contents of uranium and organic carbon and the admixture of uranium with the amorphous organic material indicates that the Mariano Lake and Ruby 1 deposits are primary-type uranium orebodies. The offset of orebodies by Laramide faults and radiometric age determinations of the ores are also consistent with a primary origin for the deposits. Late Tertiary oxygenated ground waters locally modified original chemical and mineralogical characteristics of the ores in part by leaching some uranium. Secondary uranium minerals precipitated from the partly leached primary ores are sparse in the mine area; such recycled uranium appears to represent an insignificant proportion of the total uranium in the Smith Lake district.

Economic Geology↗
USGS1983

Role of fluid mixing and fault-related sulfide in the origin of the Ray Point uranium district, south Texas

The Lamprecht and Felder deposits are roll-type deposits hosted by the Miocene Oakville Sandstone. Four distinct stages of FeS 2 mineral formation are recognized: (1) a generation of isotopically light (delta 34 S < -20 per mil) preore pyrite, (2) a generation of isotopically light (delta 34 S < -20 per mil) marcasite that formed mostly before, but also during ore deposition, (3) a postore generation of isotopically heavy pyrite (delta 34 S > 0 per mil), and (4) a yet later generation of marcasite with isotopically light sulfur close to the values of stages 1 and 2. Complex zone of mixing between saline brines migrating up the fault from depth and oxygenated meteoric water recharging at the outcrop. Bacterial activity.

Economic Geology↗
USGS1983

Opening of the Red Sea: Constraints from a palaeomagnetic study of the As Sarat volcanic field, south‐western Saudi Arabia

Four stratigraphic sections through alkali basalt flows of Oligocene to Miocene age (29‐24 Ma) in the As Sarat volcanic field, south‐western Saudi Arabia, were sampled for palaeomagnetic study. After systematic alternating‐field demagnetization, 42 magnetically acceptable flows (139 samples) yield a mean direction of magnetization of =355.3°, =15.2° (α=4.3°), which defines a palaeomagnetic pole at 78.8°N, 247.8°E. Of these acceptable flows, 24 are normally, and 18 reversely, magnetized. Part of one section was apparently erupted during the early phases of a polarity reversal of the Earth's field. The mean direction derived from the 24 normally magnetized flows is significantly different (after inverting 180°) from that derived from the 18 reversed flows, supporting the hypothesis of a displaced dipole source for the Earth's field in late Oligocene to early Miocene time. A comparison of the results from As Sarat with palaeomagnetic results from upper Tertiary rocks in Africa indicates that the Red Sea has opened 12° (estimated ±8° at the 95 per cent confidence level). These results indicate that most of the counter‐clockwise rotation of Arabia relative to Africa occurred since the eruption of the As Sarat volcanics, in disagreement with proposed models for main‐stage Red Sea spreading from late Eocene to early Oligocene time. The palaeomagnetic results also indicate that the Arabian Peninsula was more equatorial in late Oligocene to early Miocene time than it is now, by an amount compatible with the opening of the Gulf of Aden

As Sarat volcanic field↗
USGS1982

Post-depositional alteration of titanomagnetite in a Miocene sandstone, south Texas (U.S.A.)

Petrographic and geochemical studies have yielded information on the time-space relationships of the post-depositional alteration of detrital titanomagnetite (Ti-mt) in fine- to medium-grained sandstone from unoriented core samples (taken below the water table at depths of 30–45 m) of the Miocene Catahoula Sandstone, south Texas. Aqueous sulfide introduced from sour gas reservoirs along a growth fault into part of the Catahoula shortly after deposition resulted in the replacement at the periphery of Ti-mt grains by iron disulfide (FeS 2 ) minerals. Remnants of Ti-mt in cores of the partly sulfidized grains show no evidence of earlier hematitic oxidation. After sulfidization, part of the sandstone body was invaded by oxygenated groundwaters flowing down a shallowly inclined (1°) hydrologic gradient. The boundary between oxidized and reduced facies is clearly defined by the distribution of ferric and ferrous iron minerals, and the concentrations of Mo, U, and Se. In oxidized (light-red) strata that had not been previously subjected to sulfidic-reducing conditions but that are correlative with strata containing FeS 2 minerals, Ti-mt has been partly to entirely replaced pseudomorphously by hematite to form martite. The absence of hematitic alteration of Ti-mt in the reduced facies is strong evidence that martite in the oxidized facies formed after deposition.

Texas↗