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151.
152.
Rock fragment distributions and regolith evolution in the Ouachita Mountains,Arkansas, USA 总被引:2,自引:0,他引:2
Rock fragments in the regolith are a persistent property that reflects the combined influences of geologic controls, erosion, deposition, bioturbation, and weathering. The distribution of rock fragments in regoliths of the Ouachita Mountains, Arkansas, shows that sandstone fragments are common in all layers, even if sandstone is absent in parent material. Shale and sandstone fragments are produced at the bedrock weathering front, but the shale weathers rapidly and intact fragments are rare in the solum. Sandstone is weathered from ridgetop outcrops and transported downslope. Some of these fragments are moved downward, by faunalturbation and by transport into pits associated with rotting tree stumps. Upward movement by treethrow is common, resulting in a net concentration of rocks near the surface. However, the highest fragment concentrations are in the lower regolith, indicating active production at the weathering front. The regolith is a dynamic feature, reflecting the influences of vertical and horizontal processes, of active weathering at the bedrock interface, and of surficial sediment movements. The role of trees in redistributing rock fragments suggests that significant regolith mixing occurs over time scales associated with forest vegetation communities, and that forest soils have likely been extensively mixed within Holocene and historic time. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
153.
W. Crawford Elliott Stephen G. Osborn Vanessa J. O'Brien R. Douglas Elmore Michael H. Engel J. Marion Wampler 《Journal of Geochemical Exploration》2006,89(1-3):92
In this study, we test a burial model for diagenetic illite and whether illitization is related to the acquisition of a chemical remanent magnetization (CRM). A prefolding, reversed polarity, secondary magnetization is found in carbonate concretions in diagenetically altered Upper Cretaceous rocks in the disturbed belt of Montana. Stratigraphically equivalent unaltered rocks to the east do not contain the magnetization. The magnetization could be a CRM or a thermoviscous remagnetization. The pole position is generally consistent with the Early Tertiary K–Ar ages of diagenetic illite–smectite formed under thrust loading, but it is not constrained well enough for a precise age comparison. If this magnetization is a CRM, it could be related to illitization. Alternatively, if the prefolding CRM had been acquired prior to thrust loading then the magnetization is not related to illitization. Geochemical studies are currently underway to test if the magnetization could be related to fluid alteration. 相似文献
154.
C. C. Kuranz H.-S. Park B. A. Remington R. P. Drake A. R. Miles H. F. Robey J. D. Kilkenny C. J. Keane D. H. Kalantar C. M. Huntington C. M. Krauland E. C. Harding M. J. Grosskopf D. C. Marion F. W. Doss E. Myra B. Maddox B. Young J. L. Kline G. Kyrala T. Plewa J. C. Wheeler W. D. Arnett R. J. Wallace E. Giraldez A. Nikroo 《Astrophysics and Space Science》2011,336(1):207-211
The National Ignition Facility (NIF) is capable of creating new and novel high-energy-density (HED) systems relevant to astrophysics. Specifically, a system could be created that studies the effects of a radiative shock on a hydrodynamically unstable interface. These dynamics would be relevant to the early evolution after a core-collapse supernova of a red supergiant star. Prior to NIF, no HED facility had enough energy to perform this kind of experiment. The experimental target will include a 340 ??m predominantly plastic ablator followed by a low-density SiO2 foam. The interface will have a specific, machined pattern that will seed hydrodynamic instabilities. The growth of the instabilities in a radiation-dominated environment will be observed. This experiment requires a ??300?eV hohlraum drive and will be diagnosed using point projection pinhole radiography, which have both been recently demonstrated on NIF. 相似文献
155.
Aneesh Soman Thorsten Geisler Frank Tomaschek Marion Grange Jasper Berndt 《Contributions to Mineralogy and Petrology》2010,160(6):909-930
A natural, altered zircon crystal from an alkaline pegmatite from the Zomba–Malosa Complex of the Chilwa Alkaline Province
in Malawi has been studied by a wide range of analytical techniques to understand the alteration process. The investigated
zircon shows two texturally and chemically different domains. Whereas the central parts of the grain (zircon I) appear homogeneous
in backscattered electron images and are characterised by high concentrations of trace elements, particularly Th, U, and Y,
the outer regions (zircon II) contain significantly less trace elements, numerous pores, and inclusions of thorite, ytttrialite,
and fergusonite. Zircon II contains very low or undetectable concentrations of non-formula elements such as Ca, Al, and Fe,
which are commonly observed in high concentrations in altered radiation-damaged zircon. U–Pb dating of both zircon domains
by LA-ICPMS and SHRIMP yielded statistically indistinguishable U–Pb weighted average ages of 119.3 ± 2.1 (2σ) and 118 ± 1.2
(2σ) Ma, respectively, demonstrating that the zircon had not accumulated a significant amount of self-irradiation damage at
the time of the alteration event. Electron microprobe dating of thorite inclusions in zircon II yielded a Th–U-total Pb model
age of 122 ± 5 (2σ) Ma, supporting the age relationship between both zircon domains. The hydrothermal solution responsible
for the alteration of the investigated zircon was alkaline and rich in CO3
2−, as suggested by the occurrence of REE carbonates and CO2-bearing fluid inclusions. The alteration of the crystalline, trace element-rich zircon is explained by an interface-coupled
dissolution-reprecipitation mechanism. During such a process, the congruent dissolution of the trace element-rich parent zircon
I was spatially and temporally coupled to the precipitation of the trace element-poor zircon II at an inward moving dissolution-precipitation
front. The driving force for such a process was merely the difference between the solubility of the trace element-rich and
-poor zircon in the hydrothermal solution. The replacement process and the occurrence of mineral inclusions and porosity in
the product zircon II is explained by the thermodynamics of solid solution-aqueous solution systems. 相似文献
156.
Martitia P. Tuttle Robert H. Lafferty Margaret J. Guccione Eugene S. Schweig Neal Lopinot Robert F. Cande Kathleen Dyer-Williams Marion Haynes 《Geoarchaeology》1996,11(6):451-480
Prehistoric earthquake-induced liquefaction features occur in association with Native American occupation horizons in the New Madrid seismic zone. Age control of these liquefaction features, including sand-blow deposits, sand-blow craters, and sand dikes, can be accomplished by extensive sampling and flotation processing of datable materials as well as archaeobotanical analysis of associated archaeological horizons and pits. This approach increases both the amount of carbon for radiocarbon dating and the precision dating of artifact assemblages. Using this approach, we dated liquefaction features at four sites northwest of Blytheville, Arkansas, and found that at least one significant earthquake occurred in the New Madrid seismic zone between A.D. 1180 and 1400, probably about A.D. 1300 ± 100 yr. In addition, we found three buried sand blows that formed between 3340 B.C. and A.D. 780. In this region where very large to great earthquakes appear to be closely timed, archaeology is helping to develop a paleoearthquake chronology for the New Madrid seismic zone. © 1996 John Wiley & Sons, Inc. 相似文献
157.
Nordaustlandet is located in the northeastern part of the Svalbard archipelago, within the northernmost reach of the West Spitsbergen Current. This current transports Atlantic water to the Arctic Ocean along the western and northern Svalbard margins. This region is well-suited for reconstructing the history of changing Atlantic water inflow to the Arctic Ocean. We studied the marine sediment core HH12-04-GC from Rijpfjorden. Benthic foraminiferal assemblages and sedimentological data are combined to reconstruct the palaeoenvironment of the fjord from the end of the last local deglaciation to the late Holocene. The local deglaciation, between 11.3 and 10.6 cal ka bp , was dominated by active glacier calving processes, associated with a strong inflow of Atlantic water. This led to the establishment of glaciomarine conditions. The Holocene was initially characterised by a relatively stable and warm environment associated with a strong contribution of Atlantic water. Glaciomarine influence progressively decreases after 9.7 cal ka bp and the Atlantic water contribution increases. The late Holocene displayed a similar environment to today, with the influence of glaciomarine conditions and limited Atlantic water inflow. These results confirm that Atlantic water inflows made a continuous contribution to northern Nordaustlandet throughout the postglacial period. 相似文献