An epeiric ramp: low-energy, cool-water carbonate facies in a Tertiary inland sea, Murray Basin, South Australia   总被引：1，自引：0，他引：1  

Jeff J. Lukasik  Noel P. James  Brian McGowran  & Yvonne Bone 《Sedimentology》2000,47(4):851-881

The Murray Supergroup records temperate‐water carbonate deposition within a shallow, mesotrophic, Oligo‐Miocene inland sea protected from high‐energy waves and swells of the open ocean by a granitic archipelago at its southern margin. Rocks are very well preserved and exposed in nearly continuous outcrop along the River Murray in South Australia. Most facies are rich in carbonate silt, contain a background assemblage of gastropods (especially turritellids) and infaunal bivalves, and are packaged on a decimetre‐scale defined by firmground and hardground omission surfaces. Bioturbation is pervasive and overprinted, resulting in rare preservation of physical sedimentary structures. Facies are grouped into four associations (large foraminiferan–bryozoan, echinoid–bryozoan, mollusc and clay facies) interpreted to represent shallow‐water (<50 m) deposition under progressively higher trophic resource levels (from low mesotrophy to eutrophy), and restricted marine conditions from relatively offshore to nearshore regions. A large‐scale shift from high‐ to low‐mesotrophic conditions within lower Miocene strata reflects a change in climate from wet to seasonally dry conditions and highlights the influence terrestrially derived nutrients had upon this shallow, land‐locked sea. Overall, low trophic resource levels during periods of seasonally dry climate resulted in a deepening of the euphotic zone, a widespread proliferation of foraminiferan photozoan fauna and a relatively high carbonate productivity. Inshore, heterozoan facies became progressively muddier and restricted towards the shoreline. In contrast, periods of wet climate led to rising trophic resource levels, resulting in a shallowing of the euphotic zone, a decrease in epifaunal and seagrass cover and widespread development of a mostly heterozoan biota dominated by infaunal echinoids. Rates of carbonate production and accumulation were relatively low. The Murray Basin is best described as an epeiric ramp. Wide facies belts developed in a shallow sea on a low‐angled slope reaching many hundreds of kilometres in length. Grainy shoal and back‐barrier facies were absent. Internally generated waves impinged the sea floor in offshore regions and, because of friction along a wide and shallow sea floor, created a low‐energy expanse of waters across the proximal ramp. Storms were the dominating depositional process capable of disrupting the entire sea floor.  相似文献   

广西某大桥桩基方案三维弹塑性有限元分析     

许平  冯紫良 《上海国土资源》1999,(4):14-18

通过三维弹塑性有限元计算程序对处于复杂地质条件中某大桥桩基进行分析,考虑了初始地应力的影响,计算出了承台、桩顶和桩底的位移,以及承台、桩、土及岩石的应力值,并绘出了相应的应力等值线图,对基岩受力后的稳定性给出了评价。  相似文献   

Geomagnetic long-term secular variations in Italian Lower Cretaceous shallow-water carbonates     

D. H. Tarling  M. Iorio  & B. D'Argenio 《Geophysical Journal International》1999,137(3):713-722

The remanent magnetic properties of an 88  m bore core are unrelated to either the dolomite content or the sedimentological textures and are considered to be carried primarily by biogenetic magnetite that was cemented in during very early diagenesis. Individual readings represent time intervals of c . 720 ± 32  yr and, after 40  mT partial demagnetization, they provide an almost continuous record of averaged geomagnetic secular variations over a period of some 3.17  Myr. The magnitude of directional secular variation is twice that of the present day, despite being smoothed, and the secular variations appear to grade into polarity transitions, suggesting no difference in their mechanisms. The rates of change in direction between subjacent levels in the core have a log-normal distribution which extends smoothly beyond 90° and has a median value of 13°/700  yr, the same as for unsmoothed European secular variation during the last 2000  yr. The intensity of remanence, after 40  mT partial demagnetization, appears to provide a reasonable approximation to geomagnetic field intensity. This tends to be weaker when the direction is moving faster, reflecting averaging, but is unrelated to the distance of the vector from the mean direction; that is, it depends on the rate of change and not on the virtual pole latitude. The virtual poles, after correction for tectonic rotations about horizontal and vertical axes, have latitudes that form a strongly platykurtic Fisherian distribution, while their longitudes have a circular distribution on which are superimposed two Gaussian peaks, 180° apart. This bore core thus provides detailed information of smoothed geomagnetic secular variation in the Lower Cretaceous (127 ± 3  Ma) which shows clear regularities in behaviour, some related to changes in the Earth's orbital parameters.  相似文献   

Orthogonality and mean squares of the vector fields given by spherical cap harmonic potentials     

F. J. Lowes 《Geophysical Journal International》1999,136(3):781-783

It is well known that the vector fields derived from spherical harmonics are orthogonal over the sphere. It is now shown that the vector fields derived from spherical cap harmonics are orthogonal over the cap, to the same extent as the cap potentials are, and expressions are given for their mean squares.  相似文献   

中国东南沿海平原晚第四纪超浅层生物气藏盖层研究   总被引：2，自引：0，他引：2  

林春明  王彦周 《高校地质学报》1999,5(1):92-99

根据晚第四纪地层的物性特征,力学性质及烃浓度值实验资料和孔隙水压力及其消散历时现场试验结果表明,覆盖下切河谷之上的的浅海相灰色淤泥质粘土层为区域盖层,下切河人的河漫滩相灰－灰黑色粘土及淤泥质粘土层为局部盖层。粉细砂、粉砂、粉砂质粘土、淤泥质粘土和粘土的粒径依次变小,比表面积依次增大,使其孔隙率、孔隙比、天然含水量及孔隙水压力值依次增大。粘土的压缩系数、塑性指数、液性指数及抗剪强度最大,其抗变形能力相对最弱,可压缩相对最大．相对封闭性最好;而淤泥质粘土、粉砂质牯土的封闭性次之。随着埋藏深度的增加．盖层的封闭性增强。盖层的封闭机理主要为物性封闭、烃浓度封闭和孔隙水压力封闭,后者可能对生物气藏保存起着最主要的封闭作用。  相似文献   

Observations of plasma density structures in association with the passage of traveling convection vortices and the occurrence of large plasma jets     

C. E. Valladares  D. Alcaydé  J. V. Rodriguez  J. M. Ruohoniemi  A. P. Van Eyken 《Annales Geophysicae》1999,17(8):1020-1039

We report important results of the first campaign specially designed to observe the formation and the initial convection of polar cap patches. The principal instrumentation used in the experiments comprised the EISCAT, the Sondrestrom, and the Super DARN network of radars. The experiment was conducted on February 18, 1996 and was complemented with additional sensors such as the Greenland chain of magnetometers and the WIND and IMP-8 satellites. Two different types of events were seen on this day, and in both events the Sondrestrom radar registered the formation and evolution of large-scale density structures. The first event consisted of the passage of traveling convection vortices (TCV). The other event occurred in association with the development of large plasma jets (LPJ) embedded in the sunward convection part of the dusk cell. TCVs were measured, principally, with the magnetometers located in Greenland, but were also confirmed by the line-of-sight velocities from the Sondrestrom and SuperDARN radars. We found that when the magnetic perturbations associated with the TCVs were larger than 100 nT, then a section of the high-latitude plasma density was eroded by a factor of 2. We suggest that the number density reduction was caused by an enhancement in the O⁺ recombination due to an elevated T_i, which was produced by the much higher frictional heating inside the vortex. The large plasma jets had a considerable (>1000 km) longitudinal extension and were 200–300 km in width. They were seen principally with the Sondrestrom, and SuperDARN radars. Enhanced ion temperature (T_i) was also observed by the Sondrestrom and EISCAT radars. These channels of high T_i were exactly collocated with the LPJs and some of them with regions of eroded plasma number density. We suggest that the LPJs bring less dense plasma from later local times. However, the recent time history of the plasma flow is important to define the depth of the density depletion. Systematic changes in the latitudinal location and in the intensity of the LPJs were observed in the 2 min time resolution data of the SuperDARN radars. The effect of the abrupt changes in the LPJs location is to create regions containing dayside plasma almost detached from the rest of the oval density. One of these density features was seen by the Sondrestrom radar at 1542 UT. The data presented here suggest that two plasma structuring mechanisms (TCVs and LPJs) can act tens of minutes apart to produce higher levels of density structures in the near noon F-region ionosphere.  相似文献   

Multi-instrument probing of the polar ionosphere under steady northward IMF     

S. E. Pryse  A. M. Smith  L. Kersley  I. K. Walker  C. N. Mitchell  J. Moen  R. W. Smith 《Annales Geophysicae》1999,18(1):90-98

Observations are presented of the polar ionosphere under steady, northward IMF. The measurements, made by six complementary experimental techniques, including radio tomography, all-sky and meridian scanning photometer optical imaging, incoherent and coherent scatter radars and satellite particle detection, reveal plasma parameters consistent with ionospheric signatures of lobe reconnection. The optical green-line footprint of the reconnection site is seen to lie in the sunward plasma convection of the lobe cells. Downstream in the region of softer precipitation the reverse energy dispersion of the incoming ions can be identified. A steep latitudinal density gradient at the equatorward edge of the precipitation identifies the general location of an adiaroic boundary, separating the open field lines of polar lobe cells from the closed field of viscous-driven cells. Enhancements in plasma density to the south of the gradient are interpreted as ionisation being reconfigured as it is thrust against the boundary by the antisunward flow of the viscous cells near noon. Each of the instruments individually provides valuable information on certain aspects of the ionosphere, but the paper demonstrates that taken together the different experiments complement each other to give a consistent and comprehensive picture of the dayside polar ionosphere.On sabbatical leave from Artic Geophysics, University Courses on Svalbard, N-9170 Longyearbyen, Norway  相似文献   

The Great Dun Fell Experiment 1995: an overview     

K. N. Bower  T. W. Choularton  M. W. Gallagher  R. N. Colvile  K. M. Beswick  D. W. F. Inglis  C. Bradbury  B. G. Martinsson  E. Swietlicki  O. H. Berg  S. -I. Cederfelt  G. Frank  J. Zhou  J. N. Cape  M. A. Sutton  G. G. McFadyen  C. Milford  W. Birmili  B. A. Yuskiewicz  A. Wiedensohler  F. Stratmann  M. Wendisch  A. Berner  P. Ctyroky  Z. Galambos  S. H. Mesfin  U. Dusek  C. J. Dore  D. S. Lee  S. A. Pepler  M. Bizjak  B. Divjak 《Atmospheric Research》1999,50(3-4)

During March and April of 1995 a major international field project was conducted at the UMIST field station site on Great Dun Fell in Cumbria, Northern England. The hill cap cloud which frequently envelopes this site was used as a natural flow through reactor to examine the sensitivity of the cloud microphysics to the aerosol entering the cloud and also to investigate the effects of the cloud in changing the aerosol size distribution, chemical composition and associated optical properties. To investigate these processes, detailed measurements of the cloud water chemistry (including the chemistry of sulphur compounds, organic and inorganic oxidised nitrogen and ammonia), cloud microphysics and properties of the aerosol and trace gas concentrations upwind and downwind of the cap cloud were undertaken. It was found that the cloud droplet number was generally strongly correlated to aerosol number concentration, with up to 2000 activated droplets cm⁻³ being observed in the most polluted conditions. In such conditions it was inferred that hygroscopic organic compounds were important in the activation process. Often, the size distribution of the aerosol was substantially modified by the cloud processing, largely due to the aqueous phase oxidation of S(IV) to sulphate by hydrogen peroxide, but also through the uptake and fixing of gas phase nitric acid as nitrate, increasing the calculated optical scattering of the aerosol substantially (by up to 24%). New particle formation was also observed in the ultrafine aerosol mode (at about 5 nm) downwind of the cap cloud, particularly in conditions of low total aerosol surface area and in the presence of ammonia and HCl gases. This was seen to occur at night as well as during the day via a mechanism which is not yet understood. The implications of these results for parameterising aerosol growth in Global Climate Models are explored.  相似文献   

Tidal pumping and biogeochemical processes: Dissolution within the tidal capillary fringe of eogenetic coastal carbonates     

J.D. Gulley  D. Breecker  M. Covington  S. Cooperdock  J. Banner  P.J. Moore  A. Noronha  C. Breithaupt  J.B. Martin  J. Jenson 《地球表面变化过程与地形》2020,45(11):2675-2688

Growing evidence suggests microbial respiration of dissolved organic carbon (DOC) may be a principal driver of subsurface dissolution and cave formation in eogenetic carbonate rock. Analyses of samples of vadose zone gasses, and geochemical and hydrological data collected from shallow, uncased wells on San Salvador Island, Bahamas, suggest tidally varying water tables may help fuel microbial respiration and dissolution through oxygenation. Respiration of soil organic carbon transported to water tables generates dysaerobic to anaerobic groundwater, limiting aerobic microbial processes. Positive correlations of carbon dioxide (CO₂), radon-222 (²²²Rn) and water table elevation indicate, however, that tidal pumping of water tables pulls atmospheric air that is rich in oxygen, and low in CO₂ and ²²²Rn, into contact with the tidal capillary fringe during falling tides. Ratios of CO₂ and O₂ in vadose gas relative to the atmosphere indicate this atmospheric oxygen fuels respiration within newly-exposed, wetted bedrock. Deficits of expected CO₂ relative to O₂ concentrations indicate some respired CO₂ is likely removed by carbonate mineral dissolution. Tidal pumping also appears capable of transferring oxygen to the freshwater lens, where it could also contribute to respiration and dissolution; dissolved oxygen concentrations at the water table are at least 5% saturated and decline to anaerobic conditions 1–2 m below. Our results demonstrate how tidal pumping of air to vadose zones can drive mineral dissolution reactions that are focused near water tables and may contribute to the formation of laterally continuous vuggy horizons and potentially caves. © 2020 John Wiley & Sons, Ltd.  相似文献   

Messinian carbonate and alluvial fan sedimentation in Alonnisos Island,Greece: sedimentary response to basement controls,inversion tectonics and climatic fluctuations     

G. Poulimenos  P. Karkanas 《Geological Journal》1998,33(3):159-175

A marginal marine carbonate environment, giving away to an alluvial one, was established during Messinian time on Alonnisos Island, the footwall upland of the Southern Marginal Fault of the Sporades Basin (SMFS). Analysis of the evolving depositional systems, with emphasis on their sedimentation processes, faulting patterns and palaeopedological factors, has permitted an interpretation of the simultaneous controls of tectonism and climate. The carbonate sediments were deposited in a shallow marine environment formed along a faulted continental margin under warm and semi-arid climatic conditions. Faulting consisted of NE-trending dextral reverse faults and NW-trending strike slip faults, produced by WNW-directed compression. The basement structural elements affected the spatial distribution of the offshore and shoreface facies, whereas fifth-order cycles of sea-level change were responsible for the development of metre-scale, shallowing-up cycles. The compressional structures were subsequently reactivated by NNE extension. This tectonic inversion, together with a global sea-level fall, triggered alluvial fan sedimentation. Fan sedimentation was disrupted by long periods of non-deposition and soil formation under warm climatic conditions. Three distinct units are recognized in the fan: a lower unit consisting of clast-poor debris flows, attributed to semi-arid–humid periods; an intermediate unit of clast-rich sheetfloods and channel flows, deposited during arid periods; and an upper unit consisting of matrix-rich sheetfloods related to a return to semi-arid–humid conditions. We interpret that the water-flow processes responsible for deposition were most prevalent on fans of arid and semi-arid climates, whereas debris-flow processes were more typical of climates with higher rainfall. As the extension proceeded during the Plio-Quaternary time, the main tectonic activity of the Sporades Basin was taken up by the SMFS causing significant footwall uplift. Due to this process, Alonnisos Island was elevated above the Pliocene highstand and became an area starved of Quaternary sedimentation. Copyright © 1998 John Wiley & Sons, Ltd.  相似文献