bluuebunny/crossref_metadata_2025_split huggingface dataset:
Flow, sediment transport and bedform dynamics over the transition from dunes to upper‐stage plane beds: implications for the formation of planar laminae
Authors: BRIDGE JOHN S. , BEST JAMES L. 
Published: June 2006

ABSTRACTPreliminary results are reported from an experimental study of the interaction between turbulence, sediment transport and bedform dynamics over the transition from dunes to upper stage plane beds. Over the transition, typical dunes changed to humpback dunes (mean velocity 0–8 ms‐1, depth 01 m, mean grain size 0.3 mm) to nominally plane beds with low relief bed waves up to a few mm high. All bedforms had a mean length of 0.7–0.8 m. Hot film anemometry and flow visualization clearly show that horizontal and vertical turbulent motions in dune troughs decrease progressively through the transition while horizontal turbulence intensities increase near the bed on dune backs through to a plane bed. Average bedload and suspended load concentrations increase progressively over the transition, and the near‐bed transport rate immediately downstream of flow reattachment increases markedly relative to that near dune crests. This relative increase in sediment transport near reattachment appears to be due to suppression of upward directed turbulence by increased sediment concentration, such that velocity close to the bed can increase more quickly downstream of reattachment. Low‐relief bedwaves on upper‐stage plane beds are ubiquitous and give rise to laterally extensive, mm‐thick planar laminae; however, within such laminae are laminae of more limited lateral extent and thickness, related to the turbulent bursting process over the downstream depositional surface of the bedwaves.
Cat Island platform, Bahamas: an incipiently drowned Holocene carbonate shelf
Authors: DOMINGUEZ LAURA L. , MULLINS HENRY T. , HINE ALBERT C. 
Published: June 2006

ABSTRACTAnalyses of high resolution, seismic reflection profiles and surface sediment samples indicate that the Cat Island shelf is presently in an incipiently drowned state. This small carbonate bank is characterized by a thin (<4 m), coarse‐grained, relict sediment cover, along with limited reef development, and a relatively deep (20–30 m) margin indicating that it has been unable to ‘keep‐up’ with Holocene sea‐level rise.Early flooding at relatively high rates of sea‐level rise (4 m kyr‐1, 5–8 × 103 yr BP) in conjunction with small bank size and relatively low elevation, led to a reduced rate of carbonate accumulation and incipient drowning. The shelf edge currently lies beneath the zone of maximum carbonate production and exposes the interior shelf to open marine conditions which may result in permanent drowning if it is unable to ‘catch‐up’ with continued sea‐level rise. Sediment facies patterns are largely oriented perpendicular or oblique to the shelf edge and appear to be controlled by shelf circulation patterns focused by bank‐margin reentrants.In comparison with most of the northern Bahamas, the Cat Island shelf was flooded earlier and at relatively higher rates of Holocene sea‐level rise which led to selective drowning, implying that carbonate platforms need not drown synchronously over widespread areas as commonly thought. The potential rock record of this incipient drowning event would be a thin, open‐marine sand sheet of highly degraded cryptocrystalline and aggregate grains associated with poorly developed reefs.
Mixed‐water and hydrothermal dolomitization of the Pliocene Shirahama Limestone, Izu Peninsula, central Japan
Authors: MATSUMOTO RYO , IIJIMA AZUMA , KATAYAMA TETSUYA 
Published: June 2006

ABSTRACTThe early Pliocene Shirahama Limestone is a grainstone‐packstone principally composed of fragments of algae, bryozoa, and echinoderm and subordinate volcanic rocks. The limestone was variously dolomitized and the regional distribution of dolomite is patchy. Dolomite occurs as isolated crystals filling pores, moulds, and solution vugs, and mosaic aggregates replacing bioclasts. Calcite occurs as rim and pore‐filling sparry cements, and as calcareous skeletons. Isotopically, the dolomites are classified into a heavy oxygen group (−2 to − 3.5%0 PDB) and a light oxygen group (−5.5 to − 7.5%0 PDB). Calcite associated with heavy oxygen dolomite has δ18O of − 6.5 to −8.5%0 PDB, whereas those associated with light oxygen dolomite have a wide range from −7.5 to −14%0 PDB. Calcite in dolomite‐free limestone has an oxygen isotopic composition of −2 to −8.5%0 PDB. Textures, chemistry, and isotopic evidence indicate that heavy oxygen calcite formed in freshwater, and heavy oxygen dolomite in a meteoric‐marine mixture of 10–30% seawater. Light oxygen calcite and dolomite precipitated from modified hydrothermal fluids at approximately 30–65°C.Petrographic features, and both isotopic and chemical evidence suggest that the Shirahama Limestone was exposed to freshwater soon after deposition. Subsequently blocky calcite precipitated (Stage I). The limestone was locally submerged in the meteoric‐marine mixture due to gradual subsidence or eustatic movement. This led to the precipitation of heavy oxygen, zoned dolomite and dolospar (Stage II). Hydrothermal alterations occurred in the area a few Myr ago, and related hydrothermal fluids and mixed meteoric‐hydrothermal waters caused dedolomitization of some zoned dolomite, partial dissolution of vuggy dolomite, precipitation of limpid dolomite and recrystallization of some earlier dolomites (Stage III). Zeolites were also precipitated from these fluids. Finally, the Shirahama Limestone was exposed again to freshwater and sparry calcite precipitated to plug some of the remaining pores (Stage IV).
Erg margin of the Permian White Rim Sandstone, SE Utah
Authors: CHAN MARJORIE A. 
Published: June 2006

ABSTRACTThe Permian White Rim Sandstone of the Canyonlands National Park, Utah, contains a wide variety of sedimentary structures and features that largely result from stages in erg migration and marine influence on an erg margin. Three spatially distinct lithological and depositional facies are recognized and can be distinguished as informal units within the formation. The aeolian dune facies is composed predominantly of fine‐grained cross‐stratified sandstone of the White Rim erg. This facies is the most widespread and comprises the bulk of the formation. Within the aeolian dune facies are small subfacies that represent interdune deposits. A sheet sand facies, composed of parallel‐bedded sandstone, makes up a significant part of the lowest part of the White Rim Formation. This facies appears to have been the precursor or leading (progradational) edge to the main erg system. The final facies is a reworked or veneer facies of rippled to disturbed sandstone that is localized in its extent. It is restricted to the upper few metres of the formation and is transitional in some places to the Triassic Moenkopi Formation. This veneer facies contains many structures which indicate marine reworking as well as periods of desiccation or subaerial exposure.Some previous interpretations of the White Rim Sandstone have tended to classify the whole formation as one depositional setting. It is clear that at the margin of a sand sea, as shown in the White Rim Sandstone, there are transitional facies due to the interactions with other environments. Additionally, variation in the stratigraphic relationships of facies can be related to stages of erg migration. Erg margin deposits preceded central erg development. Erg initiation occurred during a probable relative sea level low. Sea level influence is recorded at the top of the formation because erg termination accompanied a relative sea level high with cut‐off of sand supply. Transgression of the Permian Kaibab Sea over the White Rim erg was probably the main process in preservation of original dune topographic relief. Sea level fluctuations also may have affected distribution of facies and the complexities of structures at the erg margin. Subsequent fluvial reworking of the veneer facies may have obliterated Late Permian features during lowest Triassic Moenkopi deposition.
Textural characteristics and genesis of the aeolian sediments in the Kuwaiti desert
Authors: KHALAF FIKRY 
Published: June 2006

ABSTRACTSeveral types of aeolian deposits have been recognized in Kuwait: (a) smooth sand sheets that resemble desert floor sand, (b) immobile sands that include rugged vegetated sand sheets and wadi fill deposits, and (c) mobile sands that form active sand sheets and sand dunes. Simple size frequency curves illustrate the genetic relationship between the various aeolian sediment types. The four size parameters, namely, mean size, sorting, skewness and kurtosis, were calculated. Scatter plot diagrams of sorting versus mean size and sorting versus kurtosis are effective in differentiating smooth sand sheet deposits from dune sands. Active sand sheet deposits can also be recognized because they are usually located between the two end members–smooth sand sheets and dune sands. Size parameters change with location regardless of their types. Coarsening and positive skewness usually increase downwind. Mineralogical and textural characteristics of the aeolian deposits in Kuwait revealed that they are mostly derived from the lower Mesopotamian muddy flood plain deposits, the sand fraction of the Al‐Dibdibba gravelly deposits and the disintegrated material from calcretic and gypcretic duricrusts. Distribution of depositional and deflational areas indicates that the northern desert of Kuwait is characterized by a positive sand budget, whereas the southern desert has a negative sand budget.
The early diagenetic origin of Lower Carboniferous mottled limestones (pseudobreccias)
Authors: SOLOMON STEPHEN T. 
Published: June 2006

ABSTRACTDark mottles are a prominent and widespread feature of the regressive, cyclic, shallow marine limestones which form the late Asbian succession in many parts of Britain.The colour difference which defines mottles in outcrop is caused by distinct but often subtle petrographic differences in the limestone fabric. Specifically, mottles contain a light brown coloured opaque inclusion‐rich calcite spar with characteristic dull brown luminescence; ‘mottle spar’. Outside mottle margins this calcite spar is absent, with clear inequant blocky cements forming the pore filling phase. ‘Mottle spar’ comprises a fabric of irregular crystals predominantly 5–40 μm in diameter, with more regular crystals up to 100 μm diameter often occurring in intraparticle and large interparticle pores. Under cathodoluminescence, ‘mottle spar’ displays crystal morphologies and growth patterns which indicate that both localized neomorphism and patchy cementation contributed to mottle formation.Cathodoluminescence cement stratigraphy shows that ‘mottle spar’ pre‐dates all other major pore filling cements in the local Asbian succession, but post‐dates marine micritization. ‘Mottle spar’ sharply defines the moulds of former aragonitic allochems which are now filled by the later clear, inequant spar cements. This shows that aragonite dissolution occurred after the formation of ‘mottle spar’. Mottles in calcretes contain unaltered allochems which have been protected from the effects of subaerial micritization by ‘mottle spar’, although mottles are often affected by subaerial brecciation. This evidence shows that mottles formed during early diagenesis; after marine micritization, but before dissolution of aragonite, subaerial exposure and meteoritic phreatic cementation.Mottles represented lithified patches of very low porosity which are interpreted to have formed in the marine/freshwater mixing zone, during the repeated phases of regression and emergence in the late Asbian.
The influence of bulk shape factors on settling velocities of natural sand‐sized sedimentary suites
Authors: GOLDBERY RON , RICHARDSON DAVID 
Published: June 2006

ABSTRACTSettling rates of natural sand‐size particles are influenced, to some extent, by their shapes and this may be an important factor in using settling rates to estimate grain size. In order to gauge the sensitivity of this influence, two natural sand populations from the Mesozoic Nubian Sandstones of Southern Israel, with a high probability of being similar in their bulk shape characteristics, were examined in ¼φ sieved fractions for their shape characteristics and settling rates. Fine surface features (roundness and surface roughness) were evaluated using Fourier shape analytical methods. Significant differences in bulk shape were detected and their influence on settling rates was measured empirically in a settling tube. The most marked differences were in the coarse grain sizes and, to a lesser extent, in the intermediate sizes.Sampling of raw settling data at closely‐spaced time intervals yielded high‐resolution grain size frequency plots which were usually polymodal in nature. Subtle shape contrasts, which are an important influence on settling rates, are thus an important consideration when working at this level of sensitivity.Natural sand populations which have followed a more varied provenance or process pathway could be expected to have even greater contrasts in settling rates than the samples analysed here. Thus it is recommended that the bulk shape factor should be taken into account in order to minimize errors in the conversion of settling times to grain size. An easy method, outlined in this paper, is through the establishment of an empirically derived calibration curve for each individual suite of sand undergoing analysis. Sieved ¼φ samples, derived from a split of the total composite sample undergoing analysis, forms the basis of the calibration and hence a correction factor converting sieve diameters to true diameters must be applied. In this research, nominal section diameters were obtained optically through an image analyser.
Grain fabric of a laboratory antidune
Authors: YAGISHITA KOJI , TAIRA ASAHIKO 
Published: June 2006

ABSTRACTThe grain fabric of a laboratory antidune composed of coarse‐ to medium‐grained sand shows a high‐angle upcurrent imbrication both on the upstream and downstream sides. Such a grain fabric differs from a normal dune or a megaripple, where grain alignment displays an upcurrent imbrication relative to the depositional slope in the foreset lamina and/or near parallelism with the slope surface in the toe‐set lamina. Recognition of antidunes in ancient rocks is difficult, but detailed examination of grain fabric of cross‐stratification makes it possible to distinguish antidunes or ‘backset’ bedding from normal dunes or megaripples.
Temporal trends in grain‐size measures on a linear sand dune
Authors: LIVINGSTONE IAN 
Published: June 2006

ABSTRACTWithin aeolian dune systems spatial patterns of grain‐size variation have been recognized, but little has been said about temporal changes. Increasingly it is becoming clear that linear dunes are associated with bi‐directional wind regimes which are often seasonal. In the Namib Sand Sea, where linear dunes are aligned roughly north‐south, winds blow from the west in summer and from the east in winter. In response to this regime, sand is eroded from the west slopes and deposited on the east slopes in summer, and eroded from the east slopes and deposited on the west slopes in winter. Preliminary evidence from a study of a single Namib linear dune reported here confirms that this seasonal aeolian regime induces seasonal responses in some grain‐size measurements due to the dynamics of sand transport on the dune, the characteristics of the sand source immediately upwind of the sample point and the nature of the deposit. Thus, time of sampling is crucial to the results obtained.
Injection clastic dykes in the Lower Oxford Clay (Jurassic) of central England: relationship to compaction and concretion formation
Authors: MARTILL DAVID M. , HUDSON JOHN D. 
Published: June 2006

ABSTRACTAn injection dyke of fine‐grained sandstone derived from the Kellaways Sand Formation intrudes overlying organic‐rich shales and shell beds of the Lower Oxford Clay. The dyke shows cross‐cutting relationships with early carbonate concretions, and fills uncompacted kosmoceratid ammonite shells both within the concretions and surrounding shales. Internally the dyke displays flow‐like features, and the walls show lobate flow structures. Clasts of uncompacted Lower Oxford Clay and fragments of pyrite‐rich concretions occur within the sandstone intrusions. The sandstone of the dyke was cemented by calcite identical to that precipitated in septarian cracks in the concretions. This cementation took place prior to final compaction of the Oxford Clay. The dyke has a sub‐parallel relationship to the nearby Tinwell‐Marholm fault suggesting that the dyke may be related to local tectonic events during the Middle Jurassic.
A vertical record of different tidal cyclicities: an example from the Miocene Marine Molasse of Digne (Haute Provence, France)
Authors: TESSIER BERNADETTE , GIGOT PATRICK 
Published: June 2006

ABSTRACTThe Miocene marine molasse of Digne is thought to have been deposited in a structural embayment with a gulf‐like morphology. This gulf was closed toward the alpine front and opened on to the Rhodano‐Provençal gulf, part of the Tethys.The outcrop of Auribeau, located on the northern margin of the gulf of Digne, displays a conspicuous vertical record of different tidal cyclicities indicating the importance of tidal dynamics on the Miocene marine molasse deposits. For the first time these tidal cyclicities are analysed from a vertical section (deposited by vertical accretion) and these are similar to the cyclicities described from megaripples or sand waves (deposited by lateral accretion). By means of thickness measurements and time series analysis, four main tidal cyclicities have been recognized: (1) the diurnal cyclicity (as it exists in the Atlantic Ocean); (2) the semi‐lunar month cyclicity of neap to spring; (3) the lunar month cyclicity of neap, high spring, neap to low spring and (4) the semi‐annual cyclicity of solstice to equinox.
Coarse‐grained submarine fan and slope apron deposits in a Cretaceous back‐arc basin, Antarctica
Authors: INESON J. R. 
Published: June 2006

ABSTRACTThe Cretaceous of west James Ross Island, Antarctica represents the proximal fill of a late Mesozoic back‐arc basin that was probably initiated by oblique extension during the early development of the Weddell Sea. The succession records sedimentation in two contrasting depositional systems: a laterally persistent slope apron flanking the faulted basin margin interrupted both spatially and temporally by coarse‐grained submarine fans. Slope apron deposits are dominated by thinly interbedded turbiditic sandstones and mudstones (mudstone association), interspersed with non‐channelized chaotic boulder beds, intraformational slump sheets and isolated exotic blocks representing a spectrum of mass‐flow processes from debris flow to submarine gliding. Localized sand‐rich sequences (sandstone‐breccia association) represent sandy debris lobes at the mouths of active slope chutes. The submarine fan sediments (conglomerate association) are typified by coarse conglomerates and pebbly sandstones, interpreted as the deposits of high‐density turbidity currents and non‐cohesive debris flows. Three assemblages are recognized and are suggested to represent components of the inner channelled zone of coarse‐grained submarine fans, from major fan channels through ephemeral, marginal channels or terraces to levee or interchannel environments.The occurrence of both slope apron and submarine fan depositional systems during the Early and Mid‐Cretaceous is attributed to localized input of coarse arc‐derived sediment along a tectonically active basin margin. Periods of extensive fan development were probably linked to regional tectonic uplift and rejuvenation of the arc source region; cyclicity within individual fan sequences is attributed to migration or switching of fan channels or canyons. Slope apron sedimentation was controlled largely by intrabasinal tectonics. Local unconformities and packets of amalgamated slide sheets and debris flow deposits probably reflect episodic movement on basin margin faults. Differential subsidence across the basin margin anchored the basin slope for at least 20 Myr and precluded basinward progradation of shallow marine environments.
Stream‐driven, high‐density gravelly traction carpets: possible deposits in the Trabeg Conglomerate Formation, SW Ireland and some theoretical considerations of their origin
Authors: TODD SIMON P. 
Published: June 2006

ABSTRACTWithin high‐density flood flows a prominent mechanism of gravel transport and deposition is by stream‐driven, high‐density traction carpet (with a rheology similar to grain flow). These gravel carpets are envisaged to form the basal portion of a bipartite high‐density flood flow, decoupled from an overlying sand‐ and silt‐laden turbulent flow. Several examples already documented in the literature are reviewed and an additional case from the Lower Old Red Sandstone of southwest Ireland is presented. Two mechanisms of traction carpet initiation are discussed: by rapid entrainment of gravel into suspension on rising stage, followed by settling into the gravel traction carpet at peak and falling stage; and by overconcentration of a ‘normal’, low‐density bedload. Gravel entrainment, suspension and traction carpet development are significantly easier if the flood water already carries a high concentration of sand and silt in suspension. Theoretical consideration further shows that gravelly traction carpets can be maintained in channels of relatively low gradient by the shear stress exerted by the high‐density, sand‐bearing turbulent flood flow above. This tangential shear stress is converted to dispersive pressure, which aids buoyancy and quasi‐static grain‐to‐grain contacts in the support of the clasts within the gravel carpet. The carpet is thought to have a quasi‐plastic rheology but behave much like a viscous fluid at high shear rates. Stream‐driven gravelly traction carpets are expected to produce sheet‐like units of clast‐ to matrix‐supported conglomerate, characterized by a parallel or an a(p)a(i) clast fabric. These units may be ungraded, normally or inversely graded, depending on the rate of shear, the viscosity of the flow and the celerity of deposition.