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21.
Pleistocene, multiple-growth, lacustrine oncoids from the Poacher's Point Formation, Etosha Pan, northern Namibia 总被引：2，自引：0，他引：2
The Poacher's Point Formation is located within the Etosha Pan, northern Namibia. It contains Pleistocene stromatolites, ooids and silica nodules. The stromatolites were constructed by cyanobacteria in very lowenergy lacustrine conditions. The internal structure of the oncoids is complex and subdivided into shells by unconformities. Each unconformity-bounded shell probably represents stromatolite growth during a distinct lacustrine event. Shallower conditions are represented by colloform growth and deeper water by smooth oncoid mat morphology. The unconformities represent hiatuses of unknown duration. At least six stromatolite growth shells are recognized. Silica concretions which may have developed during shallowing lake stages are also present. The lack of clastic material in the carbonate lithologies suggests that the lake waters may have accumulated from collecting rainwater or have been spring fed. The lowest bed of the succession is a saline clay which represents a seasonally flooded salt pan deposited in an arid to semi-arid climate similar to that of today. The overlying carbonates represent sedimentation during a series of transient lacustrine events, representing intermittent wetter periods. Finally, due to progressive aridity, the lacustrine succession was terminated by calcrete development. 相似文献
22.
An in-seam fan shoot was conducted in 1981 over a 300 m × 500 m panel of the Schwalbach seam at Ensdorf by a team from Prakla-Seismos AG of Hannover under contract to Saarbergwerke AG, Saarbrücken. The object was to study SH-mode propagation in the coal seam waveguide. The high quality dataset retrieved provides a general and valuable test bed with which to compare the performance of in-seam seismic velocity analysers. Five different dispersion analysers are demonstrated using the Schwalbach data. They are all based on the a priori assumption of coal seam homogeneity and isotropy. Space or time windows limit the resolution of the Fourier moving-window analyser, the migration based phase-velocity analyser, and the double Fourier transformer. The other two analysers, the maximum entropy moving-window analyser and the phase-moveout analyser, achieve noise-limited super-resolution by predicting the probable behaviour of the wavefield outside the window. The coal seam's characteristics conform to those predicted for a simplified model based on proposals by Elsen, Rüter and Schott of Westfälische Berggewerkschaftskasse, Bochum. The slight discrepancy between theoretical and actual dispersion characteristics could be reduced by increasing the model's complexity. However, there would be no material gain without testing the validity of the signal processing assumptions of seam isotropy and homogeneity. 相似文献
23.
DOWNES HILARY; MACDONALD RAY; UPTON BRIAN G. J.; COX KEITH G.; BODINIER JEAN-LOUIS; MASON PAUL R. D.; JAMES DODIE; HILL PETER G.; HEARN B. CARTER JR 《Journal of Petrology》2004,45(8):1631-1662
Ultramafic xenoliths in Eocene minettes of the Bearpaw Mountainsvolcanic field (Montana, USA), derived from the lower lithosphereof the Wyoming craton, can be divided based on textural criteriainto tectonite and cumulate groups. The tectonites consist ofstrongly depleted spinel lherzolites, harzburgites and dunites.Although their mineralogical compositions are generally similarto those of spinel peridotites in off-craton settings, somecontain pyroxenes and spinels that have unusually low Al2O3contents more akin to those found in cratonic spinel peridotites.Furthermore, the tectonite peridotites have whole-rock majorelement compositions that tend to be significantly more depletedthan non-cratonic mantle spinel peridotites (high MgO, low CaO,Al2O3 and TiO2) and resemble those of cratonic mantle. Thesecompositions could have been generated by up to 30% partialmelting of an undepleted mantle source. Petrographic evidencesuggests that the mantle beneath the Wyoming craton was re-enrichedin three ways: (1) by silicate melts that formed mica websteriteand clinopyroxenite veins; (2) by growth of phlogopite fromK-rich hydrous fluids; (3) by interaction with aqueous fluidsto form orthopyroxene porphyroblasts and orthopyroxenite veins.In contrast to their depleted major element compositions, thetectonite peridotites are mostly light rare earth element (LREE)-enrichedand show enrichment in fluid-mobile elements such as Cs, Rb,U and Pb on mantle-normalized diagrams. Lack of enrichment inhigh field strength elements (HFSE; e.g. Nb, Ta, Zr and Hf)suggests that the tectonite peridotites have been metasomatizedby a subduction-related fluid. Clinopyroxenes from the tectoniteperidotites have distinct U-shaped REE patterns with strongLREE enrichment. They have 143Nd/144Nd values that range from0·5121 (close to the host minette values) to 0·5107,similar to those of xenoliths from the nearby Highwood Mountains.Foliated mica websterites also have low 143Nd/144Nd values (0·5113)and extremely high 87Sr/86Sr ratios in their constituent phlogopite,indicating an ancient (probably mid-Proterozoic) enrichment.This enriched mantle lithosphere later contributed to the formationof the high-K Eocene host magmas. The cumulate group rangesfrom clinopyroxene-rich mica peridotites (including abundantmica wehrlites) to mica clinopyroxenites. Most contain >30%phlogopite. Their mineral compositions are similar to thoseof phenocrysts in the host minettes. Their whole-rock compositionsare generally poorer in MgO but richer in incompatible traceelements than those of the tectonite peridotites. Whole-rocktrace element patterns are enriched in large ion lithophileelements (LILE; Rb, Cs, U and Pb) and depleted in HFSE (Nb,Ta Zr and Hf) as in the host minettes, and their Sr–Ndisotopic compositions are also identical to those of the minettes.Their clinopyroxenes are LREE-enriched and formed in equilibriumwith a LREE-enriched melt closely resembling the minettes. Thecumulates therefore represent a much younger magmatic event,related to crystallization at mantle depths of minette magmasin Eocene times, that caused further metasomatic enrichmentof the lithosphere. KEY WORDS: ultramafic xenoliths; Montana; Wyoming craton; metasomatism; cumulates; minette 相似文献
24.
Seismic data are usually separated into P-waves and S-waves before being put through a scalar (acoustic) migration. The relationship between polarization and moveout is exploited to design filters that extract the desired wavetype. While these filters can always be applied to shot records, they can only be applied to a triaxial common-receiver gather in special cases since the moveout of scattered energy on the receiver gather relates to path differences between the surface shots and the scatterer while the polarization is determined by the path from scatterer to downhole geophone. Without the ability to separate wavefields before migration, a ‘vector scalar’ or an elastic migration becomes a necessity. Here the propagation of the elastic wavefield for a given mode (e.g. P-S) is approximated by two scalar (acoustic) propagation steps in a ‘vector scalar’ migration. ‘Vector’ in that multicomponent data is migrated and 'scalar’ in that each propagation step is based on a scalar wave equation for the appropriate mode. It is assumed that interaction between the wavefields occurs only once in the far-field of both the source and receiver. Extraction of the P, SV and SH wavefields can be achieved within the depth migration (if one assumes isotropy in the neighbourhood of the downhole receiver) by a projection onto the polarization for the desired mode. Since the polarization of scattered energy is only a function of scatterer position and receiver position (and not source position), the projection may be taken outside the migration integral in the special case of the depth migration of a common-receiver gather. The extraction of the desired mode is then performed for each depth migration bin after the separate scalar migration of each receiver gather component. This multicomponent migration of triaxial receiver gathers is conveniently implemented with a hybrid split-step Fourier-excitation-time imaging condition depth migration. The raytracing to get the excitation-time imaging condition also provides the expected polarization for the post-migration projection. The same downward extrapolated wavefield can be used for both the P-P and P-S migrations, providing a flexible and efficient route to the migration of multicomponent data. The technique is illustrated on a synthetic example and a single-level Walk-away Seismic Profile (WSP) from the southern North Sea. The field data produced images showing a P-P reflector below the geophone and localized P-P and P-S scatterers at the level of the geo-phone. These scatterers, which lie outside the zone of specular illumination, are interpreted as faults in the base Zechstein/top Rotliegendes interface. 相似文献
25.
On and Off the North China Craton: Where is the Archaean Keel? 总被引：67，自引：2，他引：65
FAN W. M.; ZHANG H. F.; BAKER J.; JARVIS K. E.; MASON P. R. D.; MENZIES M. A. 《Journal of Petrology》2000,41(7):933-950
Geophysical data indicate that the lithosphere beneath the NorthChina Craton (NCC) is 相似文献
26.
ROGER MASON 《Journal of Metamorphic Geology》1984,2(2):77-82
Abstract At Sulitjelma, Norway, there is a major inversion of metamorphic isograds beneath an inverted but undisrupted ophiolite. The flysch-like Furulund schist in which the inverted isograds occur is also inverted and the early folds in it are downward facing. The isograds cut across the axial surfaces of early folds and across the schistosity. These relationships are explained as the consequence of metamorphism during the progressive development of a large overfold. The inverted limb of the overfold is regarded as a major, thick, gently-dipping shear zone, separating the lower-grade, lower part of the Caledonian allochthon below from the higher-grade upper part of the allochthon above. The association between stratigraphical inversion, downward-facing of syn-schistosity folds and metamorphic inversion is explained by the progressive development of the shear zone. It is suggested that the presence of such shear zones is a common feature of orogenic belts formed by continental collision. 相似文献