DISTINCTION OF ELEMENTARY SAND POPULATIONS IN THE GIRONDE ESTUARY (FRANCE) BY R-MODE FACTOR ANALYSIS OF GRAIN-SIZE DATA     

GEORGE P. ALLEN  PATRICE CASTAING  RE KLINGEBIEL 《Sedimentology》1972,19(1-2):21-35

  相似文献   

Wave-generated structures in the Devonian lacustrine sediments of south-east Shetland and ancient wave conditions     

PHILIP A. ALLEN 《Sedimentology》1981,28(3):369-379

The most common wave-generated structures in the nearshore lacustrine sediments of the south-east Shetland basin are cosets of undulatory and unidirectional ripple cross-lamination. The undulatory lamination was produced at relatively high oscillatory flow strengths by accretion of rolling grain (post-vortex) ripples, and the unidirectional cross-sets were formed by the migration of vortex (orbital) ripples at lower strengths. Unidirectional solitary lenses were generated under moderate but discontinuous wave activity on a partly sand-starved substrate. Some lenses were reworked during periods of more prolonged wave activity. The Inman-Komar plot of near-bottom orbital diameter versus ripple spacing (λ= 0.80d₀ for small d₀, or λ= 0.65d₀ as modified by Miller & Komar) may only be used in estimating ancient wave conditions for vortex ripples with low Vertical Form Indices and small wavelengths. This laboratory based relationship (minimum d₀ conditions) is utilized in this study since wave periods in lakes are small. The estimation of ancient wave conditions suggests that the ripples were produced in water depths of up to 10 m and in most cases less than 5 m. The formative waves possessed periods of up to 3.4 sec and suggest that the lake was relatively small, perhaps of the order of 20 km wide.  相似文献   

The visualization of socio-economic GIS data using virtual reality tools     

DAVID MARTIN  GARY HIGGS 《Transactions in GIS》1996,1(4):255-266

This paper addresses two issues: the use of 'realistic' environments for the visualization of socio-economic data, and the use of virtual reality tools for the exploration of data from existing GIS databases. 'Realistic' is here used in a narrow sense to imply the reconstruction of real-world scenes in which major components of the physical environment are reconstructed to facilitate geographical visualization. A review of visualization techniques for geographically-referenced 3-D data is presented, and the translation of an existing urban GIS database into virtual reality modelling language (VRML) demonstrated for a study area in Cardiff, UK. This work shows that the export of conventional 2-D GIS data into virtual reality toolkits can be relatively straightforward, and offers exciting new avenues for visualization and exploration. We argue that some of the parameters of the true scene such as colour, building shape, and texture may be directly modified in order to represent otherwise unobservable socio-economic characteristics, in the spirit of exploratory data analysis. These issues are of particular importance in the context of current advances towards the dynamic linkage of GIS and virtual reality modelling.  相似文献   

Toward a model for airflow on the lee side of aeolian dunes   总被引：8，自引：0，他引：8  

ANDREW FRANK  GARY KOCUREK 《Sedimentology》1996,43(3):451-458

The interaction between dunes and the primary wind results in a complex pattern of secondary airflow on the lee side of dunes. From 15 dunes studied during transverse flow conditions at Padre Island in Texas, White Sands in New Mexico, and the Algodones in California, distinct flow regions can generally be recognized, with the overall flow structure comparing favourably to that proposed for subaqueous bedforms. Downwind of dunes with flow separation is a back-flow eddy that extends about four dune-brink heights downwind from the brink of the dune. Beyond the separation cell, the velocity profiles can be divided into regions based upon segments separated by ‘kinks’ in the velocity profiles. The interior is an area above the dunes of relative high wind speed but low velocity gradient. Beneath the interior is the wake, which consists of two layers. The upper wake exhibits an uppermost portion where the flow decelerates while the remainder exhibits accelerating flow, so that the overall velocity gradient decreases downwind. The lower wake exhibits low velocity gradients and wind speeds that accelerate downwind at all heights, but primarily near the top of the layer, thereby causing the velocity gradient to increase downwind. At about eight dune heights downwind, the upper and lower wakes equilibrate to a single profile with the kink between them no longer apparent. The lowest recognizable region is the internal boundary layer. It is recognized by a relatively steep velocity gradient below the wake, and never exceeds a few tens of centimetres in height for our data set. Because of acceleration and increasing shear stress within this layer, interdune flats are at least potentially erosional. Overall, the wake and internal boundary layer show a downward transfer of momentum from upper regions so that the flow recovers. Where flow separation does not occur, simple flow expansion down the lee-face causes flow deceleration.  相似文献   

Modelling of cyclic tidal rhythmites (Carboniferous of Indiana and Kansas, Precambrian of Utah, USA) as a basis for reconstruction of intertidal positioning and palaeotidal regimes   总被引：4，自引：0，他引：4  

ALLEN W. ARCHER  TROY W. JOHNSON 《Sedimentology》1997,44(6):991-1010

Analyses are presented for a variety of ancient cyclic tidal rhythmites, which exhibit well developed neap-spring tidal periods. Many such rhythmites were formed within the upper intertidal zone and exhibit truncated cycles that contain relatively few discrete lamina per neap-spring cycle. In such cases it can be difficult to determine if the originated palaeotidal system was diurnal or semidiurnal. Based on sedimentological controls observed in modern analogues, the development of cyclic tidal rhythmites can be modelled by use of predicted tidal-height information. The modelling used a 19-year-long series of tidal heights, which were subsequently used to approximate tidal velocities and rhythmite sedimentation. Modelling was based on a range of diurnal to semidiurnal regimes and comparisons were performed at a number of levels within the intertidal zone. This modelling produced a long series of simulated laminae-thickness series which could be cross-correlated with ancient laminae-thickness series measured directly from ancient rhythmites. This approach involved 1.6 × 10⁶ comparisons of each ancient rhythmite series to the series simulated from the predictive tidal-height information. The high correlations derived for such comparisons indicate that reasonable approximations to the palaeotidal systems can be made. In some cases, it is possible to determine the diurnal or semidiurnal nature of the originating palaeotidal system.  相似文献   

Sources of the Early Cretaceous Plutons in the Turtle and West Riverside Mountains, California: Anomalous Cordilleran Interior Intrusions     

ALLEN  C. M.; WOODEN  J. L.; HOWARD  K. A.; FOSTER  D. A.; TOSDAL  R. M. 《Journal of Petrology》1995,36(6):1675-1700

Ages and initial isotopic ratios of Early Cretaceous ({smalltilde}100 Ma) plutons of the Cordilleran Interior in the southernTurtle and West Riverside mountains distinguish them from LateCretaceous plutons in surrounding ranges in the eastern MojaveDesert. Furthermore, the studied plutons have isotopic and geochemicalcharacteristics more similar to plutons of Cretaceous age inthe coastal batholiths (Peninsular Ranges and Sierra Nevada)than to most Mesozoic plutons in the Cordilleran Interior. Thestudied plutons are calcic, in contrast to the mostly cak-alkalineMesozoic plutons of the eastern Mojave Desert. Distinctive isotopicsignatures of the granitoids include lower initial ⁸⁷Sr/⁸⁶Srof 0•705–0•710, ¹⁸O of +6•3 to +7•7,²⁰⁸Pb/²⁰⁴Pb of 38•3–39•5, and higher _Nd of –3•86to –9•60 than the Late Cretaceous plutons in theregion. The distinctive characteristics of these Early Cretaceousplutons are probably both location and time specific and resultfrom: (1) emplacement in a cold, untapped ‘Mojave-type’Proterozoic upper crust, (2) a significant component of basalticmagmas partially melted from the asthenosphere or subcontinentallithosphere and (3) a magmatic component derived from Proterozoic,mafic, lower crust. They interacted less with their crustalhosts than did the later, more voluminous Late Cretaceous plutons. KEY WORDS: Cordilleran Interior; Cretaceous; Mojave; pscudothron; Turtle Mountains  相似文献   

青藏高原东缘中新生代龙门山前陆盆地动力学及其与大陆碰撞作用的耦合关系   总被引：15，自引：1，他引：15  

李勇  P.A.ALLEN  周荣军  A.L.DENSMORE  M.A.ELLIS 《地质学报》2006,80(8):1101-1109

位于青藏高原东缘的龙门山前陆盆地是中国典型的前陆盆地之一。自晚三叠世以来,该盆地充填了厚度大于1万余米的海相至陆相沉积物,以不整合面为界可将其划分为6个构造层序,根据几何形态将构造层序区分为两种类型,即楔状构造层序和板状构造层序,其中晚三叠世、晚侏罗世、晚白垩世—古近纪构造层序为楔状构造层序,其余为板状构造层序。研究结果表明楔状构造层序为逆冲构造负载的产物,板状构造层序为走滑剥蚀卸载的产物。本次以晚三叠世前陆盆地为典型的楔状前陆盆地开展了逆冲构造负载系统的弹性挠曲动力学模拟,以晚新生代龙门山前陆盆地为典型的板状前陆盆地开展了与走滑剥蚀卸载系统的弹性挠曲动力学模拟,并计算了龙门山构造负载系统向扬子克拉通的推进速率,结果表明龙门山造山楔的推进速率在早期较快(如晚三叠世最大推进速率达15mm/a),晚期较慢(如晚侏罗世、晚白垩世—古近纪最大推进速率仅为6.7mm/a)。进而推测龙门山幕式逆冲作用的构造驱动力来自于青藏高原中生代以来的基麦里大陆加积碰撞和印度与亚洲板块碰撞作用,其中晚三叠世楔状构造层序是羌塘板块与亚洲大陆碰撞的产物,晚侏罗世楔状构造层序是拉萨板块与亚洲大陆碰撞的产物,晚白垩世—古近纪楔状构造层序是科希斯坦板块、印度板块与亚洲大陆碰撞的产物。  相似文献   

THE NATURE AND ORIGIN OF BED-FORM HIERARCHIES   总被引：3，自引：0，他引：3  

J. R. L. ALLEN 《Sedimentology》1968,10(3):161-182

The bed forms generated by liquid flows over loose-grain boundaries assume only two orientations relative to flow, being either transverse or longitudinal, but range enormously in physical scale. Two types of bed form, current ripples and parting lineations, take a characteristic length (2) which is independent of boundary conditions as expressed by flow depth (d). We have that: λ^L≤ 0.025 m (parting lineations) λ^R≤ 0.60 m (ripples). The characteristic length of other kinds of bed form is correlated with flow depth, according to the equations: λ^D= 1.16D1.^1.55 (dunes) λ^A 6.3d (antidunes, Fr = 1.0) λ^SR= 1.35d^1.31 (sand ribbons) λ^M - 672d^1.11 (meander bars) λ^T= 116d² (tidal current ridges) From these relationships can be deduced the compositions of theoretical hierarchies of bed forms, where a hierarchy is defined as a series of bed forms graded as to size which in combination are adapted to a single overall flow condition. The compositions of these hierarchies, which are closely matched by naturally occurring hierarchies, are broadly related to available flow depth, general flow conditions (lower or upper regime), and availability of sediment. Bed forms are considered to be due to unstable interactions between the bed material and the over-bed flow, of liquid or grains or both. Hierarchies of bed forms arise because the quantities determining the flow are suficiently numerous that several mutually unstable combinations can exist, each combination being expressed in terms of a bed form of a characteristic physical scale and orientation relative to flow.  相似文献   

Changes in intertidal dunes during two spring-neap cycles, Lifeboat Station Bank, Wells-next-the-Sea, Norfolk (England)   总被引：1，自引：0，他引：1  

J. R. L. ALLEN  P. F. FRIEND 《Sedimentology》1976,23(3):329-346

Twice-daily observations over a fixed range were made covering three consecutive sets of spring tides. Some dune population dimensions (e.g. mean wavelength) show a long-lasting (at least weeks) alteration during the most vigorous springs, but a short-lived (tides or days only) change over the weaker ones. Other properties (e.g. mean height) experience merely short-lived alterations. The changes seem to be linked to the creation during spring tides of new dunes of an initially small size (minor dunes), which introduces new matured dunes into the populations if there is sufficient subsequent sediment transport, and to the limited ability of individual dunes to respond to changing hydraulic conditions during their life-spans. The dunes are active during only a very small proportion of each spring-neap cycle, but the bank is permanently covered by them, and cross-stratification caused by dune movement is the only internal structure. Without a knowledge of the history and dynamics of the dune populations, the quality of the environment at Lifeboat Station Bank cannot be satisfactorily assessed from the sedimentary products.  相似文献   

INTENSITY OF DEPOSITION FROM AVALANCHES AND THE LOOSE PACKING OF AVALANCHE DEPOSITS     

J. R. L. ALLEN 《Sedimentology》1972,18(1-2):105-111

  相似文献