bluuebunny/crossref_metadata_2025_split huggingface dataset:
Selective gravel entrainment and the empirical evaluation of flow competence
Authors: KOMAR PAUL D. 
Published: June 2006

ABSTRACTThe concept of flow competence is generally employed to evaluate velocities and bed stresses of river floods from the sizes of the largest sediment particles transported. For the most part, this evaluation has been empirical, combining data from a number of separate flood events in different river systems. Those data are re‐examined and compared with empirical equations for the selective entrainment of gravel from deposits of mixed sizes. It is found that the competence relationships trend counter to those obtained for selective entrainment, indicating that the competence evaluations are affected by varying degrees of selective size entrainment. Individual data sets which have been employed to establish the flow‐competence relationships either show no trend on their own or yield a trend which runs counter to the competence equation, instead being more compatible with the selective‐entrainment relationships. In most instances, the empirical competence equations greatly overestimate the hydraulics of flood flows, and it is suggested that the better established selective entrainment equations be used for competence evaluations as well. Empirical equations are available for this purpose, relating the dimensionless Shields entrainment function or the bed shear stress to the diameter of the largest grain moved and to the median diameter of the deposit as a whole.
The origin and tectonic significance of asymmetrical meander‐belts
Authors: LEEDER MIKE R. , ALEXANDER JAN 
Published: June 2006

ABSTRACTIn the Hebgen lake area of SW Montana, the Madison and South Fork Rivers occupy the northeast sides of their respective meander‐belts. The form of the abandoned meander‐loops within these asymmetric meander‐belts indicates that they were produced by gradual northeast migration of the active channels. This progressive migration can be related to tectonic tilting produced by regional extensional faulting at a high angle to the original channel‐belt axis which ran down a gently sloping outwash cone.Asymmetric channel sandstone bodies in the rock record can be recognized by the preferred dip of successive lateral accretion units in meander loops. Together with soft sediment deformation, facies and thickness variations they may indicate tectonic deformation of the sedimentary surface.
A Terra Rossa‐like paleosol complex from the Upper Jurassic of Portugal
Authors: WRIGHT V. PAUL , WILSON R. C. L. 
Published: June 2006

ABSTRACTThe basal Upper Jurassic unconformity in the Lusitanian Basin of Portugal commonly exhibits an irregular palaeokarstic surface developed in underlying Middle Jurassic carbonates. In the Serra da Arrabida area south of Lisbon, wire cut quarry faces expose a thick Upper Jurassic paleosol complex overlying fissured and brecciated limestone veneered with a calcrete crust and associated with colluvial deposits. The paleosol complex is a red mudstone with calcrete stringers which superficially resembles present‐day red Mediterranean Terra Rossa soils. A detailed micromorphological study indicates the absence of any clay illuviation in the paleosol unit, which suggests that it is not comparable to the true Terra Rossa Alfisols, but more closely resembles present‐day Aridisols. This difference from true Terra Rossa soils probably reflects formation under a drier climate, which is confirmed by the occurrence elsewhere in Portugal of evaporitic lake deposits of the same age. The paper stresses the role of soil petrography (micromorphology) in interpreting pedogenesis in paleosols.
Sediment diffusion during overbank flows
Authors: PIZZUTO JAMES E. 
Published: June 2006

ABSTRACTDistinctive overbank sediments deposited since European settlement on the floodplain of the Brandywine Creek, Pennsylvania, are used to calibrate and test a diffusion model of overbank deposition. The predictions of the model can be calibrated to reproduce the topography of the post‐settlement lithosome with an average error of 7%. The model also correctly predicts the decrease in mean grain size away from the channel. The model greatly underestimates the ability of floodwaters to transport sand away from the channel. Apparently, sand is transported across the floodplain by bedload transport and by advective suspended sediment transport as well as by diffusion.If flow duration data for 1912–1981 and the present rating curve for the Brandywine Creek at Chadds Ford, Pennsylvania, are assumed to apply throughout the post‐settlement period, the model may be used to estimate palaeohydraulic characteristics of post‐settlement floods. Calculations indicate that 212 post‐settlement floods covered the floodplain to an average depth of 1.6 m, transported an average excess suspended sediment concentration of 6200 ppm, and deposited an average thickness of 1.4 cm of sediment on levees next to the channel.
The importance of double diffusion to the settling of suspended material
Authors: GREEN THEODORE 
Published: June 2006

ABSTRACTThe vertical fingering motions which can occur when warm, salty water overlies colder, fresher water are important to the vertical flux of salt in the ocean. An analogous and not uncommon situation is that where turbid water overlies clearer and denser water. For the case of an originally sharp horizontal interface between the two water layers (associated, as an example with an interflow due to a warm, muddy river entering a stratified lake), the resulting downward sediment flux is compared with that occurring by way of gravitational settling. Rules are proposed to ascertain when the fingering effect could be important to sedimentation. For example, it seems that fingering could be of consequence in the situation where the sediment has a diameter of 2 μm and the upper layer is about 1°C warmer than the (clear) lower layer, when the upper‐layer concentration is 1 ppm. The effect of fingering on the residence time of a gravitationally stable turbid layer is calculated. Such layers may be more short‐lived than commonly thought.
Burial dolomitization and porosity development in a mixed carbonate‐clastic sequence: an example from the Bowland Basin, northern England
Authors: GAWTHORPE ROBERT L. 
Published: June 2006

ABSTRACTWidespread dolomitization and leaching occur in the Asbian to Brigantian (Dinantian) sequence of the Bowland Basin. Within this mudrock‐dominated succession, dolomite is developed in calcarenites and limestone breccia/conglomerates deposited in a carbonate slope environment (Pendleside Limestone) and also within graded quartz wackes deposited by density currents in a generally ‘starved’ basin environment (Pendleside Sandstone). The dolomitized intervals range in thickness from less than one metre to several tens of metres and have a stratabound nature.All stages of calcite cement pre‐date dolomitization and calcite veins are dolomitized. Dolomite crystals replace neomorphic spar and may also contain insoluble residues that were concentrated along stylolites. Thus dolomitization was a late stage process within the carbonate diagenetic sequence. A late‐stage diagenetic origin is also indicated within the sandstones, with dolomite post‐dating the development of quartz overgrowths.Six main textural styles of dolomite are observed: (1) scattered; (2) mosaic; (3) subhedral to euhedral rhombic; (4) microcrystalline; (5) single crystal and (6) saddle. The style of dolomite developed is dependent on the host rock mineralogy, on whether it is space‐filling or replacive and also on temperature. Chemically the dolomite varies from near stoichiometric compositions to ankeritic varieties containing up to 20 mole % FeCO3. Generally the dolomites have isotopic compositions depleted in δ18O compared to the host limestone, with similar or lighter δ13C values.Initial dolomite was of the scattered type, but with progressive replacement of the host a mosaic dolostone with a sucrosic texture was produced. There was a general increase in the Fe and Mn content and reduction in δ18O ratio of the crystals during dolomitization. Leaching is restricted to partly dolomitized horizons, where calcite, feldspars, micas, clays and, to some extent, dolomite have been leached. This has produced biomouldic and vuggy secondary porosity within the carbonates, whereas in the sandstones honeycombed, corroded and floating grains associated with oversized pores occur. Porosity within both carbonates and sandstones is reduced by ferroan dolomite/ankerite cements.Field, petrographic and chemical characteristics indicate that dolomitizing solutions were predominantly derived from the enclosing mudrocks (Bowland Shales) during intermediate/deep burial. Fluid migration out of the mudrocks would have been sided by dehydration reactions and overpressure, the fluids migrating along the most permeable horizons—the coarse grained carbonates and sandstones that are now dolomitized and contain secondary porosity.
Glauconitization of detrital silica substrates in the Barton Formation (upper Eocene) of the Hampshire Basin, southern England
Authors: HUGHES ANDREW DAVID , WHITEHEAD DAVID 
Published: June 2006

ABSTRACTUpper Eocene detrital silica grains (chert and quartz) of the Hampshire Basin display alteration and replacement fabrics by glauconite. Silica grains have etched surfaces due to glauconitization which appear green in reflected light and thin section.Quartz grains were glauconitized by surface nucleation and replacement, which spreads from the margin with progressive glauconitization, replacing the quartz grain interior. Chert grains were glauconitized by surface replacement and nucleation internally along cracks and in pores. Different forms of glauconite are associated with the two minerals; glauconite associated with quartz is generally highly‐evolved whereas glauconite associated with chert is of the evolved variety. This is interpreted as being due to different surface‐reaction control mechanisms associated with the two forms of silica. There is no evidence to suggest that glauconite evolved in stages from a nascent form.Two crystalline morphological forms of glauconite are found associated with both quartz and chert. Glauconite growing within a confined space has a laminated morphology whilst glauconite occurring on the surface has a rosette morphology.
Geometries of Gilbert‐type deltas and large channels in the Meteora Conglomerate, Meso‐Hellenic basin (Oligo‐Miocene), central Greece
Authors: ORI GIAN GABRIELE , ROVERI MARCO 
Published: June 2006

ABSTRACTThe Meteora conglomerate is a thick, coarse‐grained unit of Oligo‐Miocene age in the Meso‐Hellenic basin consisting of Gilbert‐type delta deposits (wedge‐shaped bodies) and deep channel fills. Gilbert‐type deltas prograded from the eastern margin basinward, perpendicular to the NW‐SE basin axis. Delta bodies are thicker in the distal portion, where the dip of the foresets is lower than in the proximal area. Organized and disorganized conglomerates coexist, suggesting a mixture of mass‐ and turbulent‐flow processes. The mass flow deposits seem to prevail in the uppermost part of the foresets. Channels (up to 25 m deep) are cut into the Gilbert‐type delta deposits, and have axes perpendicular to the progradational trend of the deltas. Giant bars displaying accretion of the downstream side formed inside the channels. The channels were probably cut into Gilbert‐delta deposits due to uplift of the source area or to a fall in sea‐level. Although large cross‐sets of both Gilbert delta and channels have been formed by progradation, they can be distinguished by: (i) angle of foreset bed dips, (ii) facies and textures and (iii) vertical sequences. Differences in these features aid in distinguishing Gilbert‐type deltas from in‐channel bars.
Triassic carbonate submarine fans along the Arabian platform margin, Sumeini Group, Oman
Authors: WATTS KEITH F. 
Published: June 2006

ABSTRACTThe Sumeini Group formed along the passive continental margin slope that bounded the northeastern edge of the Arabian carbonate platform. With the initial development of this passive continental margin in Oman during Early to Middle Triassic time (possibly Permian), small carbonate submarine fans of the C Member of the Maqam Formation developed along a distally steepened slope.The fan deposits occur as several discrete lenticular sequences of genetically related beds of coarsegrained redeposited carbonate (calciclastic) sediment within a thick interval of basinal lime mudstone and shale. Repeated pulses of calciclastic sediment were derived from ooid shoals on an adjacent carbonate platform and contain coarser intraclasts eroded from the surrounding slope deposits. Sediment gravity flows, primarily turbidites with lesser debris flows and grain flows, transported the coarse sediments to the relatively deep submarine fans. Channel erosion was a major source of intraformational calcirudite.Two small submarine fan systems were each recurrently supplied with calciclastic sediment derived from point sources, submarine canyons. The northern fan system retrogrades and dies out upsection. The southern fan system was apparently longer‐lived; calciclastic sediments in it are more prevalent and occur throughout the section. The proximal portions of this fan system are dominated by channelized beds of calcirudite which represent inner‐ to mid‐fan channel complexes. The distal portions include mostly lenticular, unchannelized beds of calcarenite, apparently mid‐ to outer‐fan lobes.Carbonate submarine fans appear to be rare in the geological record in comparison with more laterally continuous slope aprons of coarse redeposited sediment. The carbonate submarine fans of the C Member apparently formed by the funnelling of coarse calciclastic sediment into small submarine canyons which may have developed due to rift and/or transform tectonics. The alternation of discrete sequences of calciclastic sediment with thick intervals of ‘background’ sediment resulted from either sea‐level fluctuations or pulses of tectonic activity.
The pattern of grainfall deposition in the lee of aeolian dunes
Authors: ANDERSON ROBERT S. 
Published: June 2006

ABSTRACTA simple model for the deposition pattern in the lee of aeolian dunes is presented that relies heavily upon a recently developed understanding of aeolian saltation. Grainfall deposition at any position on the lee face is the result of all saltation trajectories that leave any point on the surface of the dune upwind of the brink with sufficient initial velocity to travel the intervening distance. The deposition rate at any position on the lee slope is obtained by integrating over all combinations of initial position and required velocity, the velocity being weighted by its probability density.The resulting calculated total deposition rate patterns show distinct maxima on the order of one to a few decimetres from the brink, beyond which deposition rates fall off roughly exponentially. An important length scale emerges that characterizes this decay with distance from the brink, the length increasing with wind velocity, and decreasing with grain diameter. It is shown that this length scale is on the order of one metre for typical grain size and wind conditions. That this is typically smaller than the length of the lee slope is what gives rise to the oversteepening and eventual avalanching of the lee sides of aeolian dunes. The position of a pivot point on the lee slope may be predicted, separating source regions from accumulation regions for grainflow avalanche deposits.The calculated patterns provide not only a means for quantitative interpretation of active and fossil dune grainfall deposits, but they provide the initial geometry for grainflow avalanches. The initial failures should coincide with the steepest gradient in grainfall deposition, slightly downslope from the grainfall maximum.
The impact process in aeolian saltation: two‐dimensional simulations
Authors: WERNER B. T. , HAFF P. K. 
Published: June 2006

ABSTRACT A critical event in the trajectory of a sand grain saltating in air is its interaction with the surface. We examine the phenomenon of grain‐bed impacts in two dimensions using a combination of dynamical computer simulations, analytical models and physical reasoning. The results indicate that the grain‐bed collisions can be treated as two‐body collisions with the bed particle assuming an effective mass greater than its true mass. Also, the presence of geometrical surface irregularities has a strong bearing on the interaction between saltating and surface grain populations, as well as on the formation of small‐scale bedforms.
Late Pleistocene subaerial debris‐flow facies of the Bow Valley, near Banff, Canadian Rocky Mountains
Authors: EYLES NICHOLAS , EYLES CAROLYN H. , McCABE A. MARSHALL 
Published: June 2006

ABSTRACTA thick late Pleistocene sequence, comprising multiple beds of massive diamict facies resting on outwash gravels, occurs along the Bow River, near Banff, Alberta. Diamicts have a simple sheet‐like geometry which dip downvalley at between 5° and 10°, with largely conformable bedding contacts. The sediments are strongly bimodal in texture, consisting of clasts supported by a silty sand matrix. Prolate clasts show a weaka‐axis alignment parallel and transverse to the trend of the Bow Valley with a weak imbrication. Clasts of soft sediment are common. The diamict sequence has a maximum thickness of 30 m, infills the valley floor over an area of approximately 12 km2and buries a channelled topography cut on the surface of underlying outwash gravels. Abandoned braided channels, veneered by windblown sand containing volcanic tephra dated at 6600 yrbp, occur on the infill surface.Diamict facies were emplaced as subaerial debris flows derived by the mixing and remobilization of large volumes of outwash and glaciolacustrine sediment. Orientation data from clasts and a few basal grooves indicate that debris was derived upvalley and from adjacent valleyside slopes. A major episode of debris flow sedimentation may have been triggered by the abrupt drainage of lakes in tributary valleys as the Bow Valley glacier retreated. Downslope resedimentation of large volumes of sediment resulted in braided river aggradation downstream along the Bow River and can be dated to between 12 000 and 10 000 yrbp. Similar conditions probably occurred during deglaciation throughout the Canadian Cordillera and many other alpine areas, where poorly‐sorted late glacial debris‐flow facies have been mistaken for tills deposited directly by glaciers. The late glacial setting identified in this paper provides a basis of comparison with ancient continental glacial facies preserved in areas of active tectonism and high relief.
A sand budget for the Alexandria coastal dunefield, South Africa
Authors: ILLENBERGER WERNER K. , RUST IZAK C. 
Published: June 2006

ABSTRACTThe sand in the Alexandria coastal dunefield is derived from the sandy beach which forms the seaward boundary of the dunefield. Sand is blown off the beach onto the dunefield by the high‐energy onshore‐directed dominant wind. The dunefield has been forming over the past 6500 years.Sand transport rates calculated from dune movement rates and wind data range from 15 to 30 m3 m ‐1 yr‐1 in an ENE direction. The sand transport rate decreases with increasing distance from the sea due to a reduction in wind speed resulting from the higher drag imposed upon the wind by the land surface. Aeolian sand movement rates of this order are typical of dunefields around the world.The total volume of sand blown into the dunefield is 375 000 m3 yr‐1. Sand is being lost to the sea by wave erosion along the eastern third of the dunefield at a rate of 45 000 m3 yr ‐1. The dunefield thus gains 330 000 m3 of sand per year. This results in dunefield growth by vertical accretion at about 1.5 mm yr‐1 and landward movement at about 0.25 m yr‐1. The dunefield is a significant sand sink in the coastal sand transport system.The rate of deposition in coastal dunefields can be 10 times as high as rates of deposition in continental sand seas. The higher rate of deposition may result from the abundant sand supply on sandy beaches, and the higher energy of coastal winds.Wind transport is slow and steady compared to fluvial or longshore drift transport of sediment, and catastrophic aeolian events do not seem to be significant in wind‐laid deposits.
Anomalous geochemical signals from phosphatic Middle Cambrian rocks in the southern Georgina Basin, Australia
Authors: DONNELLY TERRENCE H. , SHERGOLD JOHN H. , SOUTHGATE PETER N. 
Published: June 2006

ABSTRACTMiddle Cambrian rocks of the Georgina Basin contain both phosphatic and organic‐rich sediments, the former often occurring as commercially viable phosphate deposits, and the latter as minor oil shale occurrences. Both kinds of rocks occur in the Hay River Formation penetrated by Bureau of Mineral Resources (BMR) stratigraphic corehole Tobermory 14. Detailed sampling and analysis of this core has been undertaken to assess the chemostratigraphy of the Hay River Formation; to examine the possible use, as a palaeoceanographic tool, of changes in δ13C values of carbonates and kerogens; and to gain an understanding of phosphate deposition in this part of the basin.The Upper Hay River Formation was deposited in a normal marine environment characterized by oxic bottom conditions. It contains several episodes of significant trace metal enrichment which appear to indicate periodic replacement of the water body. The Lower Hay River Formation, however, was deposited in an abnormal marine environment low in sulphate and Mo and enriched in P. Its carbonate phase has positive δ13C values 1–1.5%0 enriched over the accepted Cambrian marine carbonate value of ‐0.57 ± 0.17%0. The very low sulphate concentration, Mo‐depletion and spiking of carbonate δ18O values suggests there was an influx of deeper ocean water of sufficient volume to maintain prolonged stable anoxic bottom waters. These data support a previous suggestion that phosphogenesis in the Georgina Basin was related to ‘Oceanic Anoxic Events'.
Biogenic laminar calcretes: evidence of calcified root‐mat horizons in paleosols
Authors: WRIGHT V. P. , PLATT N. H. , WIMBLEDON W. A. 
Published: June 2006

ABSTRACTLaminar calcretes are described from the Lower Carboniferous of South Wales, the Upper Jurassic of southern England and the Upper Jurassic‐Lower Cretaceous of northern Spain. They are interpreted as calcified root‐mats (horizontal root systems) and are compared with other examples in the geological record and with possible modern analogues. All three occurrences consist of virtually identical, centimetre to decimetre‐thick, locally organic carbon‐rich, laminar micrites containing up to 50% by volume of millimetre‐sized typically calcite‐filled, tubular fenestrae set in an irregular but very finely laminated matrix. It is suggested that root‐mat calcretes are probably very common in the geological record in peritidal, lacustrine margin and floodplain deposits, but owing to their crudely biogenic microstructure, they more closely resemble cryptalgal laminites than do other laminar calcretes. The recognition of such root‐mat calcretes in sedimentary sequences not only provides evidence of subaerial exposure and vegetation cover but can also indicate positions of palaeo‐water‐tables in certain circumstances.