bluuebunny/crossref_metadata_2025 huggingface dataset:
Diagenesis and formation water chemistry of Triassic reservoir sandstones from southern Tunisia
Authors: MORAD S. , ISMAIL H. N. BEN , DE ROS L. F. , AL‐AASM I. S. , SERRHINI N‐E.
Published: June 2006

The fluvial Triassic reservoir subarkoses and arkoses (2409·5–2519·45 m) of the El Borma oilfield, southern Tunisia, were subjected to cementation by haematite, anatase, infiltrated clays, kaolinite and K‐feldspar at shallow burial depths from meteoric waters. Subsequently, basinal brines controlled the diagenetic evolution of the sandstones and resulted initially in the precipitation of quartz overgrowths, magnesian siderite, minor ferroan magnesite and anhydrite. The enrichment of siderite in 12C isotope (δ13CPDB= ‐ 14·5 to ‐ 9‰) results from derivation of carbon from the thermal decarboxylation of organic matter. During further burial, the precipitation of dickite and pervasive transformation of kaolinite into dickite occurred, followed by the formation of microcrystalline K‐feldspar and quartz, chlorite and illite, prior to the emplacement of oil. Present day formation waters are Na‐Ca‐Cl brines evolved by the evaporation of seawater and water/mineral interaction and are in equilibrium with the deep burial (≤ 3·1 km) minerals. These waters are suggested to be derived from the underlying Silurian and Devonian dolomitic mudstones.
Development of deflation lag surfaces
Authors: NICKLING W. G. , NEUMAN C. McKENNA
Published: June 2006

A series of wind tunnel tests were carried out to investigate the development of deflation lags in relation to the non‐erodible roughness element concentration. Glass spheres (18 mm in diameter) were placed along the complete length of the wind tunnel working section in regular staggered arrays using three different spacings (d=18, 30 and 60 mm) and completely covered with a 0.27‐mm erodible sand. A pre‐selected free stream velocity above threshold (8m s−1) was established above the surface and the sediment transport measured at 2‐s intervals using a wedge‐shaped trap in which an electronic balance is incorporated. Throughout each test, the emerging lag surface was periodically photographed from above at two locations upwind of the trap. The photographs were electronically scanned and analysed to calculate the lag element coverage and location, as well as mean height and frontal area for each time period. Test results indicate that lag development has a profound effect on both the sediment flux and wind profile characteristics. Initially, there is an increase in sediment flux above that for a rippled sand bed because of increased erosion around and reduced kinetic energy loss in highly elastic collisions with the emerging roughness elements. With further emergence, a dynamic threshold is reached whereupon the sediment flux decreases rapidly, tending towards zero. At this point, the supply of grains to the air stream through fluid drag follows a reduction in aerodynamic roughness and, therefore, surface shearing stress. At least as important is the lesser potential for grain ejection at impact because of reduced momentum imparted from the air stream during saltation. Although recent shear stress partitioning models indicate when particle movement may commence on varying surfaces, our experimental results demonstrate that this partitioning has a further direct bearing upon the saltation flux ratio.
A box model for non‐entraining, suspension‐driven gravity surges on horizontal surfaces
Authors: DADE W. BRIAN , HUPPERT HERBERT E.
Published: June 2006

The propagation of and the deposition from a turbulent gravity current generated by the release of a finite volume of a dense particle suspension is described by a box model. The approximate model consists of a set of simple equations, a predetermined, depth‐dependent leading boundary condition and one experimentally determined parameter describing the trailing boundary condition. It yields predictions that agree well with existing laboratory observations and more complex theoretical models of non‐eroding, non‐entraining, suspension‐driven flows on horizontal surfaces.

The essential features of gravity‐surge behaviour have been observed and are captured accurately by the box model. These include the increased rate of downstream loss of flow momentum with increased particle setting velocity, the existence of maxima in the thickness of proximal deposits, and the downstream thinning of distal deposits. Our approximation for the final run‐out distance, xr, of a surge in deep water is given by xr3(g'oq3o/w2s)1/5, where g'o is the initial reduced gravity of the surge, qo the initial two‐dimensional volume, and ws the average settling velocity of the particles in the suspension. A characteristic thickness of the resulting deposit is given by φoqo/xr'where øo is the initial volumetric fraction of sediment suspended in the surge.

Our analysis provides additional insight into other features of gravity‐surge dynamics and deposits, including the potential for the thickening of currents with time, the maintenance of inertial conditions and the potential for strong feedback in the sorting of particle sizes in the downstream direction at travel distances approaching xr. Box‐model approximations for the evolution of gravity surges thus provide a useful starting point for analyses of some naturally occurring turbidity surges and their deposits.
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

Detailed 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

This 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

The 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

The 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

A 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

The 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

Carbonate 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

The 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

A 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

The 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

No 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

The 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.
Deep‐water resedimentation of anhydrite and gypsum deposits in the Middle Miocene (Belayim Formation) of the Red Sea, Egypt
Authors: ROUCHY JEAN MARIE , PIERRE CATHERINE , SOMMER FREDERIC
Published: June 2006

The Middle Miocene evaporites in the Red Sea rift were deposited within a complex system of fault‐bounded basins that were episodically active during sedimentation. Such a tectonic framework is known to be highly favourable to resedimentation processes. An offshore petroleum well in the north‐western Red Sea has cored, below a massive salt unit, an anhydrite‐bearing succession which provides an excellent opportunity to study the processes of gravity induced redeposition of Ca‐sulphates in a deep basin. Anhydrite deposits, interbedded with siliciclastic layers and thin halite layers, are composed of resedimented facies ranging from fine‐grained laminated sediments to coarse‐grained breccias. The components derive from the reworking of shelf sediments deposited initially in shallow water to supratidal settings on the surface and edges of structural highs bordering depressions: proximal siliciclastic deposits with interstitial anhydrite (cement patches, nodules) or gypsum and dolostones with early diagenetic anhydrite facies (nodular, chicken‐wire) formed in sabkha conditions, interstitially grown gypsum crystals and subaqueous gypsum crusts precipitated in hypersaline ponds, and diatom‐rich oozes formed in marine, shallow‐water conditions. The homogeneity of the stable isotope composition and petrography of sulphates argue for the initial crystallization of Ca‐sulphates within brines of the same origin and in closely interconnected sedimentary settings. The unconsolidated sediments redeposited as slope‐foot accumulations were carried both as anhydrite (nodules, soft masses, various fragments, individual grains or crystals released by disintegration of large masses) and gypsum (crystalline aggregates or single crystals) later converted to anhydrite during burial. Layers of chaotic breccia are interpreted as the result of seismic events, whereas the fine‐grained deposits could be related to redistribution by nepheloid layers of suspensions of finer grains released by disintegration of the soft anhydrite masses during downslope transport, or of in situ deposits removed by the turbiditic flows.
Frequency distribution of river bed materials
Authors: KOTHYARI U. C.
Published: June 2006

The size distribution of river bed material plays an important role in the study of fluvial hydraulic processes. It is not always easy to sample the river bed material for the determination of its size distribution, but knowledge of the frequency distribution and of bed material sizes can be used to predict its size distribution. Therefore, data on bed material size distributions of 63 streams from 10 different countries have been analysed to determine the frequency distribution which can be best fitted to them. The log normal distribution cannot be fitted to the bed material mixtures when the entire range of sizes is considered. The method of power transformation is found to normalize these mixtures. Relationships have been proposed between the parameters of the normalization and the stream slope. These relations can be used to predict the size distribution of the river bed material.
Modelling of porosity evolution and mechanical compaction of calcareous sediments
Authors: AUDET D. MARC
Published: June 2006

A continuum mechanics model for the gravitational compaction of sediments is derived by assuming that the sediments are normally pressured and in a one‐dimensional state of stress. Sediment strength is characterized in terms of effective stress laws adopted from soil mechanics. The model is a relatively simple mathematical formula that gives the porosity as a function of burial depth. The shape of the porosity profile is controlled by two mechanical parameters, the compression index and the void ratio at an effective stress of 100 kPa. The model was verified by analysing the porosity—depth data of oozes and chalk from the Ontong Java Plateau, gathered during Leg 130 of the Ocean Drilling Program. The mechanical parameters of the sediments were estimated using a least‐squares method to fit the theoretical profile to the porosity data. The theoretical profile described accurately the ooze porosity data over depth ranges of 100 m or more. However, over smaller length‐scales of 10–50 m there were systematic deviations between the theoretical porosity values and the ooze porosity data. The porosity deviations correlated with variations in the mean grain size of the sediments, due in part to changes in the foraminifera abundance. In the case of the oozes, the estimated mechanical parameters were consistent with published values obtained from one‐dimensional compression tests. In contrast, the estimated mechanical properties for the chalks differed from published values. The chalk porosities were lower than could be explained by mechanical compaction. This explanation is supported by the compressional (P‐wave) velocity data. In the chalk sections, the P‐wave velocity increases more rapidly with burial depth than it does in the ooze sections, suggesting that sediment elastic properties are increasing due to interparticle binding.
The stability of sublittoral, fine‐grained sediments in a subarctic estuary
Authors: AMOS CARL L. , SUTHERLAND T. F. , ZEVENHUIZEN J.
Published: June 2006

The erodibility of natural estuarine sediments was measured in sit along a longitudinal transect of Manitounuk Sound, Hudson Bay, using the benthic flume Sea Carousel. Sedimentation processes along the transect varied from continuous, rapid, post‐glacial sedimentation in the inner Sound, to glacial outcrops and seabed reworking of the outer Sound. The grain size and physical bulk properties reflect changes in depositional environment and correlate with sediment erosion threshold stress (τc), erosion rate (E), erosion type and still‐water mass settling rate. There was a steady increase in τc (0·8–2·0 Pa) with distance down the Sound in parallel with the decreasing sedimentation rate (0·003–0·001 m yr−1) and increasing sediment bulk density (1650–2010 kg m−3). The near‐surface friction coefficient varied up to 68° in proportion to the clay content of post‐glacial material. Glacial sediments were characterized by variable results and generally higher friction coefficients. Seabed erosion in Sea Carousel began with surface creep of loose aggregates, pellets and organic debris. This was followed by Type I bed erosion at rates that varied between 0·0002 and 0·0032 kg m−2 s−1 (mean 0·0015). Type I peak erosion rate was inversely related to applied bed shear stress (τo). Type II erosion succeeded Type I, often after a broad transitional period. Simulations of suspended sediment concentration in Sea Carousel were made using four commonly used erosion (E) algorithms. The best results were obtained using Krone's dimensionless ratio relationship: E=M(τo/τc‐1). Simulations were highly sensitive to the definition of erosion threshold with sediment depth [τc(z)]. Small errors in definition of τc(z) caused large errors in the prediction of suspended sediment concentration which far exceeded differences between the methods tested.
Secondary precipitates in Pleistocene and present cryogenic environments (Mendoza Precordillera, Argentina, Transbaikalia, Siberia, and Seymour Island, Antarctica)
Authors: VOGT THEA , CORTE ARTURO E.
Published: June 2006

Secondary precipitates (iron oxide, calcite, etc.) are currently observed in cold‐climate Pleistocene deposits. Some have micro‐ and ultrastructures quite different from precipitates of vadose, phreatic and biogenic origin, and seem to have originated by freezing processes. The microstructures of calcite coatings from a Pleistocene cryopediment in the Mendoza Pre‐Cordillera, from a Lower Pleistocene cryogenic slope deposit in Western Transbaikalia and from the present active layer in Antarctica are described. They show similar patterns: fibrous crystals often consisting of piles of platelets, some with internal holes, assembled in millimetre‐scale fringes on the lower face of clasts. Observational (mainly fabric) evidence confirms that such peculiar crystals are formed during freezing. The features are unknown in other climates and, when found in past sediments, can be diagnostic of cryogenic palaeoenvironments.
Sedimentological aspects of the Humber Group (Upper Jurassic) of the South Central Graben, UK North Sea
Authors: HOWELL JOHN A. , FLINT STEPHEN S. , HUNT CATHERINE
Published: June 2006

Complex structure, poorly understood sedimentology and poor biostratigraphic control make the Upper Jurassic Humber Group of the South Central Graben one of the least understood and most complex hydrocarbon reservoirs of the North Sea. Detailed logging of available core from 19 exploration wells has been combined with an improved understanding of the relevance of trace fossils and a recognition of important base‐level variations to provide a greatly enhanced understanding of the depositional system active within the area at that time. A new sedimentological model, based upon the distribution of facies and facies associations, illustrates that Upper Jurassic structure and consequent basin geometry were the principal controls upon the distribution of depositional environments. Rifting and second‐order transgression controlled the back‐stepping onlap patterns observed and higher frequency base‐level fluctuations controlled the internal architecture of individual sandbodies. The model presented accounts for features of these deposits that were previously considered anomalous, such as the thickness of bioturbated sandstones, paucity of foreshore deposits and complex age relationships of sands.
Sedimentology of a lacustrine fan‐delta system, Miocene Horse Camp Formation, Nevada, USA
Authors: HORTON BRIAN K. , SCHMITT JAMES G.
Published: June 2006

A 1600‐m‐thick succession of the Miocene Horse Camp Formation (Member 2) exposed in east‐central Nevada records predominantly terrigenous clastic deposition in subaerial and subaqueous fan‐delta environments and nearshore and offshore lacustrine environments. These four depositional environments are distinguished by particular associations of individual facies (14 defined facies). Subaerial and subaqueous fan‐delta facies associations include: ungraded, matrix‐and clast‐supported conglomerate; normally graded, matrix‐ and clast‐supported conglomerate; ungraded and normally graded sandstone; and massive to poorly laminated mudstone. Subaqueous fan‐delta deposits typically have dewatering structures, distorted bedding and interbedded mudstone. The subaerial fan‐delta environment was characterized by debris flows, hyperconcentrated flows and minor sheetfloods; the subaqueous fan‐delta environment by debris flows, high‐ and low‐density turbidity currents, and suspension fallout.

The nearshore lacustrine facies association provides examples of deposits and processes rarely documented in lacustrine environments. High‐energy oscillatory wave currents, probably related to a large fetch, reworked grains as large as 2 cm into horizontally stratified sand and gravel. Offshore‐directed currents produced uncommonly large (typically 1–2 m thick) trough cross‐stratified sandstone. In addition, stromatolitic carbonate interbedded with stratified coarse sandstone and conglomerate suggests a dynamic environment characterized by episodic terrigenous clastic deposition under high‐energy conditions alternating with periods of carbonate precipitation under reduced energy conditions. Massive and normally graded sandstone and massive to poorly laminated mudstone characterize the offshore lacustrine facies association and record deposition by turbidity currents and suspension fallout.

A depositional model constructed for the Horse Camp Formation (Member 2) precludes the existence of all four depositional environments at any particular time. Rather, phases characterized by deposition in subaerial fan, nearshore lacustrine and offshore lacustrine environments alternated with phases of subaerial fan‐delta, subaqueous fan‐delta and offshore lacustrine deposition. This model suggests that high‐energy nearshore currents due to deep water along the lake margin reworked sediment of the fan edge, thus preventing development of a subaqueous fan‐delta environment and promoting development of a well‐defined nearshore lacustrine environment. Low‐energy nearshore currents induced by shallow water along the
Hydrodynamics and sediment dynamics of The Wash embayment, eastern England
Authors: KE X. , EVANS G. , COLLINS M. B.
Published: June 2006

Water and sediment movement in The Wash embayment has been determined from an extensive set of data, consisting of tidal current readings and suspended sediment concentration measurements. Instantaneous and residual currents in the embayment show a distinct lateral inhomogeneity, whereas vertically the water column is almost homogenous. The central deep water area (30–40 m) of the embayment is dominated by a residual landward water movement, whilst on the margins, the residual movement is seaward. Sediment is supplied predominantly in suspension from the north, through the northern extremity of Boston Deep. Suspended sediment pathways are coincident with the spring tide water movements and the subtidal channels act as the main conduits. Approximately 6·8 × 106 tonnes yr−1 of suspended sediments are supplied to the embayment from offshore areas. Bedload sediment supply is of lesser importance, ∼ 1·4 × 104 tonnes yr−1. Whereas suspended sediment movement appears to be the dominant mode of transport throughout the embayment, bedload transport is important in reforming the sea bed into a variety of bedforms which are particularly well developed on the margins of channels and shoals.
Bedload transport of aggregated muddy alluvium from Cooper Creek, central Australia: a flume study
Authors: MAROULIS J. C. , NANSON G. C.
Published: June 2006

The ability of mud aggregates to form depositional bedforms is of considerable sedimentological importance for explaining the geomorphology of the Channel Country of central Australia as well as for understanding the depositional environment of certain argillaceous fluvial sequences in the rock record. The sediment transport and bedform development of mud aggregates from the floodplain of Cooper Creek, central Australia, was examined in a laboratory flume over a range of flow conditions. The aggregates were found to be clay‐rich (>60% clay), nonsaline (<0·02%), fine sand‐sized (mean d50=0·13 mm), low density (2300 kg m−3) and water‐stable. Three wetting rates were applied to the sediment in the laboratory prior to wet sieving to replicate various field conditions and results in three mean aggregate sizes. Immersion wetting (no tension) represents inundation of the sediment by overland flow and results in aggregates of 0·13 mm. Tension wetting at 20 and 50 mm corresponds to high‐ and low‐intensity rainfall and results in mean d50 sizes of 0·75 and 0·70 mm, respectively. Immersion wetting is the most applicable wetting mode for hydraulic transport of aggregated sediment on the Cooper Creek floodplain. Considerable variability in sediment transport rates in the field could result from differences in pre‐wetting of the aggregated sediment. The dominance of smectite in the clay mineralogy of the sediment is an important factor in the development of the aggregates; disaggregated sediment reaggregated in a laboratory after 2–3 wetting/drying cycles. In flume experiments, bedforms of aggregated mud ranging from lower‐regime plane beds to upper‐regime antidunes were observed. The aggregates moved predominantly as bedload with measured peak bedload concentrations being high compared with other flume studies. The highly mobile nature of this sediment in the field is due to the ready entrainment of low‐density aggregates in the form of self‐mulching vertisols across extensive floodplains. The occurrence of low‐sinuosity braid‐like channels on this extensive low‐gradient semi‐arid floodplain can be attributed to: (a) the passage of floodwaters across a floodplain with steeper gradients than adjacent more sinuous anastomosing channels; (b) the highly mobile nature of the low‐density sediment aggregates; (c) the ability of the aggregates to be transported as bedload; and (d) their durable nature during transport.
Mesoproterozoic deep‐water reefs from Borden Peninsula, Arctic Canada
Authors: NARBONNE GUY M. , JAMES NOEL P.
Published: June 2006

A superbly exposed stromatolite reef complex occurs in the Victor Bay Formation near Strathcona River on northern Baffin Island. Individual reefs are up to 130 m thick and nearly 1 km in length, and their development was clearly related to their position in the facies spectrum and to sea‐level dynamics. In the first sea‐level cycle, metre‐scale reefs grew amongst mid‐ramp calcarenites and outer‐ramp shales during slow sea‐level rise; a 25‐m‐thick oblate reef tract, separating mid‐ramp and outer‐ramp facies, formed during the highstand. The greatest period of reef growth was during the second sea‐level cycle. Pinnacle reefs nucleated on the karsted upper surface of the oblate reef tract and aggraded rapidly in response to rising sea‐level, producing structures with more than 75 m of depositional relief. A gradual symmetrical succession of stromatolite growth forms, from stratiform to cylindrical columns to conical columns and then back through cylindrical columns to stratiform, is mirrored by evidence in offreef deposits for deepening to a maximum flooding surface and then shallowing. The tops of these high‐standing reefs were karsted during the following regression, while dolomite ‘cryptodomes’ grew as sheets on their submerged flanks and as progradational tongues extending basinward of the reefs. Continued sea‐level fall resulted in subaerial exposure of the entire reef complex and the extensive formation of surface and subsurface karst.

These Proterozoic slope buildups are similar to Phanerozoic deep‐water reefs in size, shape, prevalence of synsedimentary lithification, presence of Neptunian dykes and in their well‐developed vertical zonation of reefbuilders. However, they differ in being constructed exclusively by stromatolites rather than being mud mounds with small skeletal elements, and in lacking halos of perireefal sand‐ and gravel‐sized calcareous debris. Their responses to changes in sea‐level were strikingly similar to those shown by their younger counterparts, and suggest that sequence‐stratigraphic concepts derived from studies of Phanerozoic reefs can also be applied to the Proterozoic.