bluuebunny/crossref_metadata_2025_split huggingface dataset:
Mean flow and turbulence structure over fixed, two‐dimensional dunes: implications for sediment transport and bedform stability
Authors: BENNETT S. J. , BEST J. L. 
Published: June 2006

ABSTRACTDetailed measurements of flow velocity and its turbulent fluctuation were obtained over fixed, two‐dimensional dunes in a laboratory channel. Laser Doppler anemometry was used to measure the downstream and vertical components of velocity at more than 1800 points over one dune wavelength. The density of the sampling grid allowed construction of a unique set of contour maps for all mean flow and turbulence parameters, which are assessed using higher moment measures and quadrant analysis. These flow field maps illustrate that: (1) the time‐averaged downstream and vertical velocities agree well with previous studies of quasi‐equilibrium flow over fixed and mobile bedforms and show a remarkable symmetry from crest to crest; (2) the maximum root‐mean‐square (RMS) of the downstream velocity values occur at and just downstream of flow reattachment and within the flow separation cell; (3) the maximum vertical RMS values occur within and above the zone of flow separation along the shear layer and this zone advects and diffuses downstream, extending almost to the next crest; (4) positive downstream skewness values occur within the separation cell, whereas positive vertical skewness values are restricted to the shear layer; (5) the highest Reynolds stresses are located within the zone of flow separation and along the shear layer; (6) high‐magnitude, high‐frequency quadrant‐2 events (‘ejections’) are concentrated along the shear layer (Kelvin‐Helmholtz instabilities) and dominate the contribution to the local Reynolds stress; and (7) high‐magnitude, high‐frequency quadrant‐4 events occur bounding the separation zone, near reattachment and close to the dune crest, and are significant contributors to the local Reynolds stress at each location. These data demonstrate that the turbulence structure associated with dunes is controlled intrinsically by the formation, magnitude and downstream extent of the flow separation zone and resultant shear layer. Furthermore, the origin of dune‐related macroturbulence lies in the dynamics of the shear layer rather than classical turbulent boundary layer bursting. The fluid dynamic distinction between dunes and ripples is reasoned to be linked to the velocity differential across the shear layer and hence the magnitude of the Kelvin‐Helmholtz instabilities, which are both greater for dunes than ripples. These instabilities control the local flow and turbulence structure and dictate the modes of sediment entrainment and their transport rates.
On wind velocity profile measurements taken in wind tunnels with saltating grains
Authors: SPIES PETER‐JOST , McEWAN IAN K. , BUTTERFIELD GRAEME R. 
Published: June 2006

ABSTRACTThis paper reviews some aspects of wind tunnel experiments on sand‐transporting winds. It follows previous papers that have discussed the influence of the outer region of the boundary layer on wind velocity measurements. This influence was quantified with the use Coles’Wake function. In this paper this correction is applied to six previously described wind velocity profiles. An attempt is made to calculate the profile parameter II from these measurements. The values found were not consistent with the expected II, which was determined by Coles for clean air flow. This value (II=0.55) was assumed to be valid in previous analyses for sand‐transporting winds. Evidence for a mutual dependency of friction velocity and profile parameter is presented and the difficulty in determining u. is pointed out. It is suggested that the constant stress region of the boundary layer should be kept large enough for measurements when Coles’Wake function is not to be used in the data analysis.
Sustained high‐density turbidity currents and the deposition of thick massive sands
Authors: KNELLER BENJAMIN C. , BRANNEY MICHAEL J. 
Published: June 2006

ABSTRACTThe origin of massive sands in turbidite successions has commonly been attributed to the rapid dumping of sand due to flow unsteadiness in collapsing, single surge‐type, high‐density turbidity currents. The general applicability of this model is questioned here, and we propose that rapid deposition of massive sands also occurs due to non‐uniformity in prolonged, quasi‐steady high‐density turbidity currents. We attempt to eliminate ambiguity in the use of the terms ‘deceleration’and ‘unsteadiness’with respect to non‐uniform sediment gravity flows, and stress that, as with any particulate current, unsteadiness is not a prerequisite of sediment deposition. We propose a mechanism of gradual aggradation of sand beneath a sustained steady or quasi‐steady current, and upward‐migration of a depositional flow boundary that is dominated by grain hyperconcentration and hindered settling. Formation of tractional structures is prevented by the absence of a sharp rheological interface between the lowest parts of the flow and the just‐formed dewatering deposit. Deposition continues as long as the downward grain flux to the depositional flow boundary is balanced by grain supply from above or from upcurrent. Massive sand deposited in this way is not, strictly, a result of ‘direct suspension sedimentation’in that it is characterized by grain interactions, hindered settling, shear and, possibly, by interlocking of grains. The thickness of the resulting massive sand bears no relation to the thickness of the parental current, and the vertical variation within the deposit may reveal little about the vertical structure of the current, even during deposition. Thin, normally graded tops or mud drapes represent the eventual waning of sustained currents.
Physical stratigraphy and sedimentology of the Late Pleistocene‐Holocene Tiber Delta depositional sequence
Authors: BELLOTTI P. , MILLI S. , TORTORA P. , VALERI P. 
Published: June 2006

ABSTRACTThe Tiber Delta lies on the passive continental margin of central western Italy. The Tiber Delta is a Late Pleistocene—Holocene sedimentary succession and constitutes an ‘incomplete’high‐frequency depositional sequence (4th or 5th order), which developed from the last post‐glacial rise in sea level to the present stillstand. It is bound by a type 1 basal unconformity which cuts Pleistocene deposits.Five depositional systems have been identified: (1) coastal barrier—lagoon; (2) shelf‐transition; (3) fluvial braided; (4) Tiber lagoonal deltas (T1, T2, T3); and (5) Tiber wave‐dominated delta. The first four systems constitute the transgressive system tract (TST) which is characterized by a retrogradational parasequences set, while the fifth system represents the highstand system tract (HST) characterized by only one parasequence (progradational). The evolution of these depositional systems has been mostly controlled by sea‐level rise, which was not continuous, but punctuated by eight minor stillstands and rapid rises. This mechanism of sea‐level change led to the formation of parasequences that developed within the depositional systems. From a palaeogeographical point of view the present Tiber Delta area was mainly characterized by a coastal barrier, lagoon and Tiber lagoonal deltas (T1, T2, T3), which migrated landwards during the sea‐level rise. Throughout the time of the present stillstand, the T3 lagoonal delta prograded rapidly into the lagoon, reaching the coastal barrier, and the lagoon was replaced first by marshes and then by the alluvial plain. Finally, the Tiber River overcame the littoral barrier supplying the coast, giving rise to the present wave‐dominated delta.
The use of Fourier descriptors in the classification of particle shape
Authors: THOMAS M. C. , WILTSHIRE R. J. , WILLIAMS A. T. 
Published: June 2006

ABSTRACTA new quantitative method for characterizing quartz grain shape is presented. The method employs a harmonic analysis based upon Fourier descriptors which is a distinct variation of the traditional and widely used Fourier series. Quartz grain images from a scanning electron microscope were ‘frame grabbed’and converted to a digitized grey‐level image. The image processing techniques of enhancement, segmentation and boundary tracking were applied to remove all features except the image boundary. This boundary was sampled at uniform intervals of are length and represented mathematically on the complex plane. In this way problems associated with the location of particle centroid and re‐entrant values were avoided. The resulting data was standardized relative to scale, rotation and starting position. Hence the discrete Fourier transform was applied using modern fast Fourier transform techniques and the modulus of the resulting harmonic amplitude used to characterize the grain shape. The technique was applied to a sample of 0–5‐m quartz grains from three distinct populations: desert quartz, beach grains (Fire Island, New York) and Brazilian crushed quartz. Whilst plots of average amplitude vs. harmonic number for each population appeared similar, discriminant analysis applied to each grain sample distinguished characteristic grain shape with an excellent degree of success. The problems of location of the centroid and re‐entrant values were eliminated. This allowed the technique to be applied to a much wider group of irregularly shaped sedimentary particles such as loess.
Fabric signature of subglacial till deformation, Breidamerkurjökull, Iceland
Authors: BENN DOUGLAS I. 
Published: June 2006

ABSTRACTThe foreland of Breidamerkurjökull, Iceland, is the only locality where tills known to have undergone subglacial deformation are exposed. Till on the foreland has a two‐tiered structure, consisting of a dilatant upper horizon c 0.5 m thick and a compact lower till; these horizons correspond to the ductile deforming A horizon and the brittle‐ductile B horizon observed below the glacier by G. S. Boulton and co‐workers. The relationship between known strain history and a variety of macrofabric elements is examined for these two genetic facies of deformation till. The upper horizon exhibits variable a‐axis fabrics and abundant evidence for clast re‐alignment, reflecting ductile flow and rapid clast response to transient strains. In contrast, the lower horizon has consistently well organized a‐axis fabrics with a narrow range of dip values, recording clast rotation into parallel with strain axes during brittle or brittle‐ductile shear. The data indicate that till strain history imparts identifiable macrofabric signatures, providing important analogues to guide the interpretation of Pleistocene tills.
Application of the trace element and isotope geochemistry of strontium to studies of carbonate diagenesis
Authors: BANNER JAY. L. 
Published: June 2006

ABSTRACTCarbonate rocks and natural waters exhibit a wide range in the concentration and isotopic composition of strontium. This wide range and the quantifiable covariation of these parameters can provide diagnostic tools for understanding processes of fluid‐rock interaction. Careful consideration of the uncertainties associated with trace element partitioning, sample heterogeneity and fluid‐rock interaction mechanisms is required to advance the application of the trace element and isotope geochemistry of strontium to studies of diagenesis, goundwater evolution, ancient seawater chemistry and isotope stratigraphy.A principal uncertainty involved in the application of Sr concentration variations to carbonate systems is the large range of experimental and empirical results for trace element partitioning of Sr between mineral and solution. This variation may be a function of precipitation rate, mineral stoichiometry, crystal growth mechanism, fluid composition and temperature. Calcite and dolomite in ancient limestones commonly have significantly lower Sr concentrations (20–70 p.p.m.) than would be expected from published trace element distribution coefficient values and Sr/Ca ratios of most modern sedimentary pore waters. This discrepancy probably reflects the uncertainties associated with determining distribution coefficient values.As techniques improve for the analytical measurement and theoretical modelling of Sr concentration and isotopic variations, the petrological analysis of carbonate samples becomes increasingly important. The presence of even small percentages of non‐carbonate phases with high Rb concentrations and high 87 Sr86 Sr values, such as clay minerals, can have significant effects on the measured 87 Sr/86 Sr values of carbonate rocks, due to the decay of 87Rb to 87 Sr. For example, a Permian marine limestone with 50 p.p.m. Sr and 1 p.p.m. Rb will have a present‐day 87 Sr/86 Sr value that is >2 × 10−4 higher than its original value. This difference is an order of magnitude greater than the analytical uncertainty, and illustrates the importance of assessing the need for and accuracy of such corrections.A quantitative evaluation of the effects of water‐rock interaction on Sr concentrations and isotope compositions in carbonates strengthens the application of these geochemical tracers. Geochemical modelling that combines the use of trace elements and isotopes can be used to distinguish between different mechanisms of water‐rock interaction, including diffusive and advective transport of diagenetic constituents in meteoric pore fluids during the recrystallization of carbonate minerals. Quantitative modelling may also be used to construct diagnostic fluid‐rock interaction trends that are independent of distribution coefficient values, and to distinguish between mixing of mineral end‐members and fluid‐rock interaction.
Diagenesis of Barremian‐Aptian platform carbonates (the Urgonian Limestone Formation of SE France): near‐surface and shallow‐burial diagenesis
Authors: MOSS STEVE , TUCKER MAURICE E. 
Published: June 2006

ABSTRACTThe diagenesis of carbonate platform sediments is controlled by the original facies and mineralogy, climate, sea‐level changes and burial history; these controls are clearly seen in the diagenesis of the Urgonian platform carbonates of SE France. Early diagenesis in the Urgonian platform included the precipitation of marine cements, dissolution of rudist shells and minor karstification. Diagenetic features produced during this phase were controlled by several falls in relative sea‐level during the Barremian to mid‐Aptian punctuating platform sedimentation, the original mineralogy of the sediment and the prevailing semi‐arid/arid climate in the region at this time. Following a relative sea‐level rise and further sedimentation, progressive burial of the platform led to minor compaction, followed by precipitation of coarse, equant, zoned to non‐luminescent, calcite cement. This cement was cut by later stylolites, suggesting a relatively shallow‐burial origin. Stable isotope (mean values ‐ 7.94%δ18O and 0.36%δ13C) and trace element (mean values of Fe 334 ppm, Mn 92 ppm and Sr 213 ppm) data suggest that these cements precipitated from meteoric fluids at temperatures slightly elevated relative to depositional temperatures.A variable thickness of replacive dolomite which occurs preferentially within the shelf‐margin facies of the lower part of the Urgonian post‐dates mechanical fracturing and chemical compaction, but pre‐dates the main phase of stylolitization. It is probable that the dolomitizing fluid was sourced by the early compaction‐driven release of connate fluids held within the underlying muddy units.The burial history of these rocks suggests that calcite cementation and dolomitization took place at relatively shallow burial depths (1–1.5 km). The overall diagenetic history of the Urgonian Limestone Formation is a reflection of the pre‐conditioning of the platform limestones by climate, sea level, tectonics and the shallow burial depths experienced by the platform during the later Mesozoic.
The motion of sediment‐water mixtures during intense bedload transport: computer simulations
Authors: JIANG ZHENHUA 
Published: June 2006

ABSTRACTA new model, which couples fluid and particle dynamics, has been developed to study the motion of the sediment‐water mixture during intense bedload transport, including the velocity profiles of both sediment and water, the roughness length of an upper plane bed and the thickness of moving sediment layers. Standard mixing length theory is used to model the motion of water above the boundary between the overlying water and the sediment‐water mixture. The turbulent flow within the moving sediment layers is described by a shear stress model, in which the effective viscosity of the flowing water is proportional to the velocity difference between the fluid and the sediment. The particle dynamics method, in which the equations of motion of each of many particles are solved directly, is applied to model the movement of sediment particles. The particle‐fluid interaction is expressed by a velocity‐squared fluid drag force exerted on each sediment particle. Both computer simulation results and theoretical analysis have shown that the velocities of both sediment and fluid during intense sediment transport decrease exponentially with depth in the top layers of a fast‐moving sediment—water mixture. The thickness of the moving sediment layers, obtained from the computer simulation results, is proportional to the shear stress, which agrees with previous experimental observations.
The application of high‐resolution sequence stratigraphy to fluvial systems: a case study from the Upper Carboniferous Breathitt Group, eastern Kentucky, USA
Authors: AITKEN JOHN F. , FLINT STEPHEN S. 
Published: June 2006

ABSTRACTThe Pennsylvanian Pikeville, Hyden and Four Corners formations of the Breathitt Group in eastern Kentucky, USA, contain six major facies associations along with a number of subassociations. These facies associations are offshore siltstone, rhythmically bedded mouthbar heteroliths, predominantly fine‐grained floodplain deposits, minor channel fills, major distributary channels and major, stacked fluvial bodies. The stacked fluvial bodies are incised into a variety of open marine and delta plain deposits, have widths of several kilometres and exhibit a range of sandy fill types. These fluvial complexes are interpreted as incised valley fills.Parasequences and parasequence sets are not identifiable. Nonetheless, it is possible to identify systems tracts on the basis of sequential position, facies associations and systematic changes in architectural style and sediment body geometries. The studied portion of the Breathitt Group comprises stacked 4th‐order sequences, which occur in lowstand, transgressive and highstand sequence sets related to the development of a lower frequency base level cycle.In the lowstand sequence set, incision associated with successive 4th‐order sequence boundaries has commonly removed all the HST and TST of the underlying sequences, such that succeeding 4th‐order incised valley fills are amalgamated. Within the transgressive sequence set, incision is at a minimum and incised valley fills tend to stack discretely with the maximum amount of fine‐grained TST and HST between them. The highstand sequence set is transitional between the lowstand and transgressive sequence sets in terms of the amount of transgressive and highstand deposits preserved. Incised valley fills tend to stack discretely.
Carbonate ramp depositional systems from a late Jurassic epeiric platform (Iberian Basin, Spain): a combined computer modelling and outcrop analysis
Authors: AURELL MARC , BOSENCE DAN , WALTHAM DAVE 
Published: June 2006

ABSTRACTNo counterparts to epeiric‐sea carbonate ramps are known in present‐day environments. This hinders the interpretation of the factors controlling the growth and evolution of these depositional settings. In this study we analyse the facies and geometries of two Jurassic examples both from outcrop study and through computer modelling. This analysis is constrained by two important features of these Oxfordian and Kimmeridgian ramps: firstly, they are very well exposed, allowing accurate reconstruction of a 200‐km section from proximal to distal ramp environments, and, secondly, a time framework for correlation, section reconstruction and modelling is provided by a well‐defined ammonite biostratigraphy. The modelling results in a synthetic stratigraphy which closely matches the reconstructed cross‐sections and, when integrated with the field study, constrains and provides additional quantitative data on the following aspects of carbonate ramp systems.Resedimentation by storms is an important process in maintaining the ramp profile through time. Down‐ramp transport distances of between 25 and 40 km are indicated from the distribution of storm beds and shallow‐water allochems and from model‐matching known stratigraphic thicknesses and geometries.Modelling sediment production within the time constraints from the ammonite biozones indicates that shallow‐water carbonate production was 1–2 orders of magnitude less than that predicted for present‐day open‐marine carbonate platforms. Deeper‐water production rates were reduced by lesser amounts. These proportionally higher, outer‐ramp production rates also help to maintain ramp geometries through time.The enigmatic slope crest of ramps is shown to result from a combination of higher, shallow‐water production and erosion rates, together with loss of accommodation during highstands and high‐stillstands in the modelled sea‐level curves.The most parsimonious modelling of the two ramp sequences comes from a relative sea‐level curve composed of a linear subsidence component superposed by 20‐ and 100‐kyr cycles on a third‐order cycle. The third‐order cycles and their timing do not correspond to those of the Exxon curve.
A Late Ordovician high‐energy temperate‐water carbonate ramp, southern Quebec, Canada: implications for Late Ordovician oceanography*
Authors: LAVOIE DENIS 
Published: June 2006

ABSTRACTThe Trenton Group (Late Ordovician), the youngest carbonate unit in the Taconic foreland basin of southern Quebec, is a tripartite unit with a distinctive coarse‐grained middle part, the Deschambault Formation.Lithofacies of the Deschambault Formation are dominated by coarse‐grained bioclastic/intraclastic limestones; finer‐grained lithofacies are ubiquitous but subordinate. The complete spectrum of lithofacies indicates sedimentation ranging from above fairweather‐ to below storm‐wave base. Skeletal components are indicative of the modern temperate‐water bryomol association. Non‐skeletal elements are represented by peloids and intraclasts. Accretion rates from areas of continuous sedimentation were low (<14 cm/103 years). From sedimentological and faunal evidence, it is proposed that the Late Ordovician Deschambault ramp was bathed by temperate waters. The model compares favourably with modern cool‐water shelves rimming the southern edge of the Australian continent.Palaeomagnetic data locate southern Quebec in a low latitudinal setting during the Late Ordovician. Upper Ordovician facies distribution in eastern Canada and progressive disappearance of some faunal provinces through Late Ordovician time are used to conclude that the initiation of the Late Ordovician glaciation that covered most of Gondwana was instrumental in easing northward movement of cold oceanic currents. This resulted in the rapid contraction of the southern hemisphere warm‐water tropical belt from a 30° latitudinal‐wide zone in the early Caradoc to a 15° zone in the late Caradoc.