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431.
Sedimentary deposits are often characterized by various distinct facies, with facies structure relating to the depositional
and post-depositional environments. Permeability (k) varies within each facies, and mean values in one facies may be several
orders of magnitude larger or smaller than those in another facies. Empirical probability density functions (PDFs) of log(k)
increments from multi-facies structures often exhibit properties well modeled by the Levy PDF, which appears unrealistic physically.
It is probable that the statistical properties of log(k) variations within a facies are very different from those between
facies. Thus, it may not make sense to perform a single statistical analysis on permeability values taken from a mix of distinct
facies. As an alternative, we employed an indicator simulation approach to generate large-scale facies distributions, and
a mono-fractal model, fractional Brownian motion (fBm), to generate the log(k) increments within facies. Analyses show that
the simulated log(k) distributions for the entire multi-facies domain produce apparent non-Gaussian log(k) increment distributions
similar to those observed in field measurements. An important implication is that Levy-like behavior is not real in a statistical
sense and that rigorous statistical measures of the log(k) increments will have to be extracted from within each individual
facies.
Electronic Publication 相似文献
432.
The sedimentary succession of a Late Jurassic (Oxfordian to basal Kimmeridgian) carbonate ramp is described and interpreted.
The study area is located in the central part of the Lower Saxony Basin in NW Germany, which forms part of the Central European
Basin. Eight well-exposed and undeformed sections of the study area (Süntel area, Wesergebirge and eastern part of the Wiehengebirge)
provide detailed information about lithofacies and lateral thickness variations. Biostratigraphically, the age of these sediments
is poorly constrained. Twenty microfacies types are recognized that can be grouped into seven facies associations: (a) strongly
bioturbated marlstones deposited near storm wave base (SWB), (b) foraminifera-rich wackestones, (c) wackestones and floatstones
with biostromes and (d) bioclastic limestones deposited between SWB and fair-weather wave base (FWWB), (e) oolitic and iron-oolitic
limestones and (f) siliciclastic sediments deposited above FWWB, and (g) lagoonal deposits. These facies associations characterize
a storm dominated shallow mixed carbonate-siliciclastic ramp. Based on facies changes, quartz content, and gamma ray logs,
the Korallenoolith Formation can be subdivided into a lower carbonate-dominated and an upper siliciclastic-dominated part,
build up by different scales of small- to large-scale deepening- and shallowing-upward cycles. A preliminary correlation of
measured outcrops of this formation is presented. 相似文献
433.
Triassic stratigraphy,facies and evolution of the Arabian shelf in the northern United Arab Emirates
Florian Maurer Roberto Rettori Rossana Martini 《International Journal of Earth Sciences》2008,97(4):765-784
This study provides a reconstruction of the Late Permian and Triassic depositional history of the Arabian shelf in the northern
United Arab Emirates based on facies analysis and foraminiferal biostratigraphy. The presented data show that sedimentation
occurred in three major sequences. From the Late Permian to Olenekian carbonates and evaporites were deposited in restricted
lagoons and tidal flats. After a hiatus, sedimentation resumed and continued until the Late Ladinian/Carnian, leading to the
deposition of a carbonate platform dominated by peritidal dolostones. A period of shelf exposure and erosion, spanning from
the Carnian to Norian, was followed by the third major sequence with sedimentation into the Early Jurassic. During this third
depositional sequence sedimentation changed from pure carbonate into mixed carbonate–siliciclastic deposits. This transition
reflects the global regression of the sea in the Late Triassic (Triasina hantkeni Zone) and the increased erosion of large parts of the Arabian hinterland. A comparison of the evolution of the Arabian shelf
in the study area with chronostratigraphic reference schemes for the Arabian Plate reveals remarkable differences in the distribution
of Middle and Upper Triassic sequences. These are most likely the result of poor biostratigraphic control on previously studied
formations in the region. 相似文献
434.
The central part of the Cygnus OB2 association was searched for the presence of short-period hot pulsators. Out of 16 O-type stars in the field, five turned out to be variable. None of the O-type variables showed short-period variations characteristic for the β Cephei-type stars. However, two β Cephei stars were found among the early B-type members of the association. Their pulsation periods are too long to be in agreement with the average association age. We note that the discrepancy can be explained by non-coeval star formation, which was already suggested by other authors from the investigation of the colour–magnitude diagrams of the association. We found 29 new variables in all fields searched by us. About half of these are Cygnus OB2 members. By means of Hα photometry, we confirmed the presence of Hα emission in two stars and found one new Be star. A weak Hα emission is probably present in all observed O-type supergiants and giants. We also give the new BVRI Hα photometry for almost 300 stars. This multicolour photometry is used to determine a dereddened colour–magnitude diagram for the association and make a reddening map of the observed field. The extinction in the field, expressed as the total absorption in the Johnson V filter, A v , ranges from 4.1 to 7.0 mag; the average A V amounts to 5.8 mag. One O-type star is probably a background object. 相似文献
435.
Noah Brosch Eran O. Ofek Elhanan Almoznino Timothy Sasseen Michael Lampton & Stuart Bowyer 《Monthly notices of the Royal Astronomical Society》1998,295(4):959-969
We analyse an ultraviolet observation toward the Coma cluster by means of the FAUST shuttle-borne telescope. We detect 56 candidate sources to a signal-to-noise ratio of 6.5. Most sources have optical counterparts in existing catalogues and are stars. 10 sources with no listed counterparts were observed at the Wise Observatory. We present identifications based on low-resolution spectrophotometry, and discuss the foreground stars and the galaxies, all in the foreground of the Coma cluster. The FAUST image yields a higher fraction of hot evolved stars than either of the North Galactic Pole or the Virgo regions, analysed previously. We identify these tentatively as cooling white dwarfs in the Coma Berenices (Mel 111) open cluster. 相似文献
436.
R. D. Jeffries D. J. James & M. R. Thurston 《Monthly notices of the Royal Astronomical Society》1998,300(2):550-560
We present CCD photometry and high-resolution spectroscopy of low-mass stars in the open cluster NGC 2516, which has an age of about 150 Myr and may have a much lower metallicity than the Pleiades. 24 probable F to early K type, single cluster members have been identified from their photometry and radial velocities, along with three possible spectroscopic binaries. The projected equatorial velocities are measured and compared with younger and older clusters. Several fast rotating late G /early K stars are seen, but all hotter stars have v e sin i < 20 km s−1 . The data are consistent with angular momentum loss models with spin-down time-scales that increase from tens of Myr for G stars to hundreds of Myr for K stars. The observed X-ray activity is consistent with the currently accepted rotation–activity paradigm. Lithium abundances are derived from the Li i 6708-Å line. The pattern of Li depletion is indistinguishable from that in the Pleiades, including a spread in the K0 stars, where the most rapid rotators suffer the least Li depletion. The observations argue in favour of either a metallicity in the range −0.1 < [Fe/H]< 0.0 for NGC 2516, or a lower metallicity and extra Li depletion through non-standard mixing modes which occurs on time-scales of only ∼ 50 Myr. Neither our low signal-to-noise ratio spectroscopy nor our photometry can constrain [Fe/H] sufficiently to decide between these possibilities. A detailed spectroscopic chemical abundance analysis is urgently required. 相似文献
437.
M. Tosi L. Pulone G. Marconi & A. Bragaglia 《Monthly notices of the Royal Astronomical Society》1998,299(3):834-844
We present CCD BVI photometry of the old open cluster Berkeley 21, one of the most distant clusters in the Galactic anticentre direction, and possibly the lowest metallicity object in the open clusters sample. Its position and metal abundance make it very important for the study of the Galactic disc. Using the synthetic colour–magnitude diagram method, we estimate values for the distance modulus ( m − M )0 = 13.4–13.6, reddening E ( B − V ) = 0.74–0.78 (with possible differential absorption), and age = 2.2–2.5 Gyr. 相似文献
438.
本文以结构分类为基础,将工作区的碳酸盐岩归纳为20种岩相,同时对比分析了每种岩相的基本沉积条件。根据岩相的相邻和共生关系,建立了7个岩相组合,并解释了各个相组的沉积环境和各岩相的微环境。根据相组在剖面上的分布特征,讨论了环境的时空演化。认为纵向上存在着两套向上变浅的旋回;横向上在大环境格局类似前提下,存在着个别环境差异。 相似文献
439.
During the Albian and Cenomanian, the Pieniny Klippen Belt Basin, a part of the Carpathian geosynclinal domain, showed a clear differentiation into an axial deepest part represented by the Pieniny and Branisko successions and two marginal zones, a southern (Nizna and Haligovce successions, in Slovakia) and a northern zone (in Poland and Slovakia) represented by the Niedzica, Czertezik and Czorsztyn successions, becoming progressively more shallow towards the north. Five palaeobathymetric foraminiferal associations have been distinguished in the axial and northern marginal zone sediments of the Klippen Basin, corresponding to: ‘A’ shelf and upper slope: relatively large proportion of nodosariids and miliolids (Czorsztyn succession); ‘B1’ middle part of slope; oligotaxic planktonic assemblage dominant (Niedzica through Branisko successions and northern part of the Pieniny succession); ‘B2’ middle part of slope: larger proportion of agglutinated foraminifers, association characteristic of sediments influenced by turbidites (submarine flyschoid channels in the Branisko succession); ‘Cl’ middle and lower parts of slope: scarce microfauna, Hedbergella and textularids dominant (Pieniny succession, middle part); ‘C2’ slope/abyssal plain transition, close to foraminiferal lysocline (probably about 3500m below sea level): scarce specimens corroded and slightly dissolved (Pieniny succession, southern part). 相似文献
440.
The palaeoenvironmental significance of trace fossil assemblages in the flysch deposits of the Upper Cretaceous of the Sistan ocean – the Sefidabeh basin in the Sistan Suture Zone SSZ in Eastern Iran – has been assessed for the first time. The Sefidabeh basin of turbidite origin consists of 10 sedimentary facies, which can be grouped into 3 facies associations (FA) representing submarine channel-related facies associations (FA1), lobe-related facies associations (FA2), distal fan-basin floor facies associations of a deep-water turbidite system (FA3). Thirty three ichnogenera, with many ichnospecies, have been identified in this deep sea succession: Alcyonidiopsis, Arthrophycus, Asterostoma, Belorhaphe, Bergaueria, Cardioichnus, Chondrites, Cosmorhaphe, Desmograpton, Gyrophyllites, Halopoa, Helminthopsis, Helminthorhaphe, Laevicyclus, Lophoctenium, Mammilichnis, Megagrapton, Multina, Nereites, Ophiomorpha, Palaeophycus, Planolites, Phycodes, Phycosiphon, Paleodictyon, Rutichnus, Scolicia, ?Strobilorhaphe, Taenidium, Teichichnus, Thalassinoides, Zoophycos and Urohelminthoida. Their distribution is clearly linked with lithofacies and depositional palaeoenvironments. Changes in trace fossil assemblages and ichnocoenoses follow different environments of the turbidity system of the submarine channel to fan system of the Sefidabeh basin and are associated with variations in environmental controlling factors. Environmental controlling factors including hydrodynamic regime, oxygen level, organic content and sedimentation rates. Ten ichnocoenoses were recognized in the facies associations of the deep-sea fan system of this study. Taking into consideration the diversity, bioturbation level, and colonization order of bioturbated beds and the obvious deepening of the deep-sea depositional system from inner to outer parts of the succession, ichnocoenoses can express a bathymetric trend from shallower to deeper parts, and from higher-to-lower hydrodynamic condition of deep-sea fan systems of the Sefidabeh basin. This study reveals important sedimentological and ichnological features of turbiditic systems in deep sea settings of Iran and permits the development of predictive models for the palaeoenvironmental significance of trace fossil assemblages that can be readily translated to analogous depositional systems in the surface/subsurface. 相似文献