bluuebunny/crossref_metadata_2025_split huggingface dataset:
Laboratory measurements of pivoting angles for applications to selective entrainment of gravel in a current
Authors: LI ZHENLIN , KOMAR PAUL D. 
Published: June 2006

ABSTRACTImportant to grain entrainment by a flowing fluid is the pivoting angle of the grain about its contact point with an underlying grain. A series of experiments has been undertaken to determine how this angle depends on grain shape (rollability and angularity), on the ratio of the size of the pivoting grain to those beneath, and on factors such as imbrication. The experiments involved gravel‐sized spheres (ball‐bearings and marbles), natural pebbles selected for their approximately triaxial ellipsoid shapes, and angular crushed basalt pebbles. The pivoting angles for these grains were measured on an apparatus consisting of a board which can be progressively inclined, the angle of the board being equal to the pivoting angle at the instant of grain movement.The pivoting angles of spheres showed reasonable agreement with a theoretically derived equation, showing much better agreement than in previous studies which utilized sand‐sized spheres. A series of measurements with spheres ranging from sand to gravel sizes reveals that the pivoting angles decrease with increasing particle size. Our results are therefore consistent with the earlier studies limited to sand‐size spheres. The cause of this size dependence is unknown since moisture and electrostatic binding can be ruled out. Similar size dependencies are also found for the ellipsoidal pebbles and angular gravel.The experiments with ellipsoidal pebbles demonstrated a strong shape dependence for the pivoting angle, being a function of the ratio of the pebble's smallest to intermediate axial diameters. This ratio controls the grain's ability to roll and pivot; with small ratios of these diameters the pebbles tended to slide out of position, whereas with ratios closer to unity (circular cross‐section) true pivoting took place and the angles were smaller. Experiments with flat pebbles placed in an imbricated arrangement yielded much larger angles than when the pebbles lay in a horizontal position, the pivoting angle being increased approximately by the imbrication angle. The angular crushed gravel also required high pivoting angles, apparently due to interlocking of the grains resulting from their angularity.Other factors being equal, the measurements of pivoting angles demonstrate that the order of increasing difficulty of entrainment is spheres, ellipsoidal grains, angular grains, and imbricated grains. The results obtained here make possible the quantitative evaluation of these shape effects on grain threshold, as well as evaluation of the selective entrainment of grains from a bed of mixed sizes.
The behaviour of intertidal sandwaves during neap‐spring tide cycles and the relevance for palaeoflow reconstructions
Authors: TERWINDT JOOST H. J. , BROUWER MARGUERITE J. N. 
Published: June 2006

ABSTRACTVariations in migration distances and shape characteristics of sandwaves in relation to flow conditions were studied on the Ossenisse intertidal shoal in the Westerschelde estuary. The purpose was to analyse bedform behaviour, to establish the threshold and the time lags involved, to find differences in two‐ and three‐dimensional sandwaves and to determine the implications for palaeoflow reconstructions.Sandwave migration is well correlated with the peak depth‐averaged flow velocity of the dominant tide. Thus the latter parameter may be estimated from the thickness of the tidal bundles. Other flow parameters such as shear velocity, Chezy C or roughness length do not show a correlation with the migration and cannot be used in palaeoflow analysis.Flow depth does not correlate with sandwave height or with length. Consequently, neither sandwave height and length nor set height and length can be used for palaeoflow depth determination.Sandwaves start moving when the peak dominant flow velocity exceeds 0.5–0.6 m s−1, and appreciable changes in shape occur at 0.75–0.8 m s −1. Complete reversal of sandwaves is accomplished if both the dominant and subordinate peak depth‐averaged current velocities exceed 0.85 m s−1.Two‐ and 3‐D sandwaves appeared to have different stability fields in the velocity‐depth diagram and in the diagram of the Froude number versus the depth‐grain‐size ratio. In addition the distinction between 2‐D and 3‐D sandwaves appeared to be related to a variability in current direction during periods of appreciable sand transport. There are also differences in sedimentary structures between the two types of sandwaves.
Black shales and carbon isotopes in pelagic sediments from the Tethyan Lower Jurassic
Authors: JENKYNS HUGH C. , CLAYTON CHRISTOPHER J. 
Published: June 2006

ABSTRACTDetailed sampling and analysis of Jurassic pelagic limestones and marls from Italy, Hungary and Switzerland have enabled construction of an isotope stratigraphy across the Pliensbachian‐Toarcian boundary with resolution to the zonal level. The oxygen‐isotope record is unremarkable. The carbon isotopes, however, show two positive excursions: one, relatively minor, during the Pliensbachian, margaritatus Zone, subnodosus Subzone, the other, more major, during the Toarcian. early falciferum Zone, where a maximum δ13C value of 4·52%PDB is attained. These intervals are known to be favoured periods of organic‐rich sedimentation in diverse parts of the globe and the isotopic excursions are interpreted as a response to abnormally high rates of storage of organic carbon in the sedimentary record. A comparable phenomenon has been documented from the Cenomanian‐Turonian boundary in the Cretaceous where it has been referred to the influence of an ‘Oceanic Anoxic Event’.Some Italian sections spanning this Lower Jurassic interval contain organic‐rich shales in the falciferum Zone; the isotopic signatures from their included, locally manganiferous carbonate betray a considerable diagenetic overprint and they cannot therefore be incorporated in a composite isotopic curve. Carbon isotopes from the organic carbon itself are extremely negative, falling to –33δPDB and, in one section examined in detail, correlate with the calcium‐carbonate content of the shales; they may reflect a partial change to a non‐calcified planktonic biota during deposition of this lime‐poor interval, possibly responding to upwelling and increased fertility of near‐surface waters. The onset of upwelling may have been as early as spinatum‐tenuicostatum Zone time, that is, at the Pliensbachian‐Toarcian boundary.
A glove box for the fine‐scale subsampling of sediment box cores
Authors: EDENBORN HARRY M. , MUCCI ALFONSO , BELZILE NELSON , LEBEL JEAN , SILVERBERG NORMAN , SUNDBY BJØRN 
Published: June 2006

ABSTRACTThe construction and operation of a glove box which allows the fine‐scale vertical subsampling of sediment box cores under a low oxygen atmosphere is described.
Microfacies and geochemistry of Middle Jurassic algal limestones from Scotland
Authors: ANDREWS JULIAN E. 
Published: June 2006

ABSTRACTBathonian algal limestones from the Duntulm Formation of the Great Estuarine Group, western Scotland, are subdivided into three microfacies: cryptalgal laminites, thrombolites and porostromate‐micrites. The distribution of early diagenetic, carbonate spherulites and cement fringes is fabric selective. Spherulites are common in the thrombolitic microfacies, whereas cement fringes were best developed upon algal nodules with porostromate microstructure. Syndiagenetic micrites, spherulites and cement fringes have stable carbon isotopic ratios indicative of abundant, organic‐derived HCO3 incorporation (δ13C values of −0·05 to −15‰). Enhanced magnesium concentrations in these fabrics, relative to neomorphic and burial‐diagenetic spar cements, suggest an original Mg‐calcite mineralogy, probably with precipitation under the influence of microbial systems. Brecciated, early‐diagenetic fabrics and localized pockets of calcite pseudomorphs after gypsum were probably formed during subaerial exposure. Sulphate supply was probably from storm‐derived, saline lagoon‐water washed on to supralittoral stromatolitic flats. These inundations also allowed the temporary establishment of green algae and encrusting foraminifera. The combined geological and geochemical information suggests a palaeoenvironment somewhat similar to Recent, sub‐tropical humid zone, calcareous algal marshes.
Berm development on a monsoon‐influenced microtidal beach
Authors: THOMAS K. V. , BABA M. 
Published: June 2006

ABSTRACTThe mechanisms of berm development along a microtidal‐high energy beach is examined. Such a beach with medium‐sized sand and monsoon wave‐controlled profile at Valiathura, south‐west coast of India, is selected for this study. The waves which very rarely fall below 1 m, often exceed 4 m during the monsoon period of May to October. The erosion‐accretion pattern of the beach shows a cyclicity and the berm development is mainly due to the onshore migration and welding of longshore bars on to the beach following the monsoon rough season. The stages of berm development in the present microtidal beach are more or less similar to the model presented by Hine for a mesotidal case, except for the following intermediate additional stages. The longshore bar develops due to the erosion of beach when the wave steepness was above 0·04, gets flattened when it falls below 0·04, and then reforms nearer to the shoreline as a swash bar. This reformed bar gets divided and the inner bar gets welded on to the beach, followed by the outer bar developing the berm. During the onshore migration of the longshore bar and berm development the beach face becomes partially reflective with the surf scaling parameter, εb between 2·5 and 33. The inshore is dissipative with the inshore surf scaling parameter, εs≫33. The offshore side of the longshore bar is partially reflective with its surf scaling parameter, εbar between 2·5 and 33. The breakers are spilling or plunging. Vertical growth of the berm is mainly due to the changes in swash‐limit caused by the variations in wave steepness, breaker height and type. Vertical growth stops when the beach‐face attains equilibrium with the grain size‐wave energy relationship, and a wave steepness below 0·02 helps to sustain this state.
Estimation of some aeolian saltation transport parameters: a re‐analysis of Williams' data
Authors: JENSEN JENS LEDET , SØRENSEN MICHAEL 
Published: June 2006

ABSTRACTA new method for analysing observed aeolian sand transport rate profiles of the kind obtained by Williams is presented. The method involves a mathematical model of aeolian saltation. Detailed information about the saltation process can be calculated from the transport rate profile by means of this model. The method is used to perform a re‐analysis of Williams' trap data. Among the main findings of this analysis is that the grain borne shear stress appears to be a smaller fraction of the total shear stress than assumed by Bagnold & Owen in their theories of aeolian saltation. Other findings are that the probability distribution of the jump height of the grains does not depend much on the wind speed once the saltation is established, and that the vertical component of the mean launch velocity decreases with the grain size. It is approximately inversely proportional to the grain diameter. Our estimates of the landing angles indicate that estimates of the impact angles obtained from photographically recorded trajectories are too small due to biased sampling. The influence of grain shape on the transport characteristics is mainly due to changes in the grains' ability to jump when hitting the bed. It is found that angular grains have a lower mean jump height than spherical grains.
Pyrite replacement of mollusc shells from the Lower Oxford Clay (Jurassic) of England
Authors: FISHER IAN ST JOHN 
Published: June 2006

ABSTRACTReplacement of originally aragonite mollusc shells by pyrite commonly occurs in the Lower Oxford Clay. Petrographic studies show the shells to have constituted complex microenvironments in the sediment. A range of replacement textures is found showing a variable amount of solution of the original aragonite. Three distinct textures were found in crushed pyrite‐replaced ammonite shells from heavily pyritized concretions.(1) A texture reflecting the original shell structure due to the replacement of the organic shell‐matrix by pyrite.(2) An ovoid texture seen at several stages of replacement reflecting processes occurring at discrete centres of sulphate reduction.(3) Euhedral crystals lining cracks and fractures in the shell.Three types of replacement are found in small gastropods and bivalves from shell bed, some of which may relate to those seen in the ammonites.(1) Replacement of organic shell‐matrix by pyrite preserving good shell‐microstructure.(2) Replacement showing outwardly good preservation of morphological features but inwardly only the gross structure, such as growth lines, is preserved.(3) Replacement of the shell in a matrix of euhedral pyrite leaving only lines of carbonate inclusions marking the margins of the shell.The replacement textures and types appear to be dependent on the initial structure of the shell and the access of iron and sulphate into the shell.Early stages of replacement appear to proceed by pyrite formation within the organic matrix of the shell, with little or no solution of the carbonate, this produces textures which faithfully mimic the original shell microstructure. It is thought that the lack of carbonate solution is due to a limited availability of iron, brought about by the less intensively reducing nature of the sediment. Later stages of replacement are promoted by the cracking and fracturing of the shell and are, generally, not as faithful to the original shell structure. This is due to the greater availability of iron as the sediment becomes more reducing with burial.
Conditions favourable for the formation of warm‐climate aeolian sand sheets
Authors: KOCUREK GARY , NIELSON JAMIE 
Published: June 2006

ABSTRACTAeolian sand sheets are areas of aeolian sand where dunes with slipfaces are generally absent. Sand sheets are ubiquitous to modern, warm‐climate sand seas, generally occurring marginal to dune fields, although they may exist within the interior of a sand sea or independent of a dune field. Sand‐sheet deposits are recognized in ancient aeolian sequences, where they may account for significant accumulations of low‐angle aeolian stratification. We suggest that the occurrence of sand sheets instead of dunes indicates that conditions are outside the range within which dunes form or that one or more factors interfere with dune development while also favouring the accumulation of sand sheets. A study of six modern sand sheets in North America (located at Great Sand Dunes, Gran Desierto, Dumont, Algodones, Padre Island, and Colorado River delta) indicates that the factors favourable for sand‐sheet development are: (1) a high water table, (2) surface cementation or binding, (3) periodic flooding, (4) a significant coarse‐grained sediment population, and (5) vegetation. These factors are reflected in the nature of stratification and the accessory features of sand‐sheet accumulations within the areas of modern sand sheets as well as in their ancient counterparts in the Triassic Dolores and Pennsylvanian‐Permian Rico formations.
Submergent shorelines in the SW Pacific: evidence for an episodic post‐glacial transgression
Authors: CARTER R. M. , CARTER L. , JOHNSON D. P. 
Published: June 2006

ABSTRACT High resolution seismic profiles, supported where possible by radiocarbon dates and regional stratigraphic data, indicate that the last post‐glacial transgression in the SW Pacific was episodic, comprising major stillstands punctuated by rapid rises in sea‐level.On the terrigenous continental shelf east of South Island, New Zealand, a succession of shorelines (S8‐S1) are recognized, as follows: S8 =c. −113 m, 18,000 yr BP; S7 =c. −88 m, 17,000 yr BP; S6 =c. −75 m, 15,000 yr BP; S5 =c. −56 m, 12,000 yr BP; S4 =c. −46 m, 11,000 yr BP; S3 =c. −28 m, 9,500 yr BP; S3a =c. −24 m, 9,000 yr BP; S2 =c. −9 m, 7,500 yr BP; S1 = 0 m, 6,500 yr BP.With the exception of S8, and possibly S2, the shorelines are associated with wedges of sediment, the size and presence of which imply that (1) sea‐level stabilized at some shorelines for a considerable period of time (up to 1‐2,000 yr); and (2) the intervening rises of sea‐level, estimated to have been at least 10‐12 m 103 yr−1, were too rapid to allow the reworking of the wedges into a transgressive sediment sheet, as favoured in some current models.On the Great Barrier Reef shelf, off Queensland Australia, shorelines S8‐S1 have also been recognized, with a further shoreline feature S4a occurring at c. ‐39 m. Shorelines S1a (0 m/0 yr BP), Sib (+ 2‐3 m/6,000 yr BP) and Sic (0 m/6,500yr BP) are recognized as discrete aspects of the post‐6,500 yr BP sea‐level behaviour in north‐eastern Australia. The rapid rise in sea‐level, at least between shorelines S5 (12,000 yr BP) and S3 (9,500 yr Bp), is known to have outpaced reef growth, causing in situ drowning of reefs located along the deeper shorelines. All modern reefs so far drilled and dated began their development at or above S3 (‐28 m, 9,500 yr BP).Some of the shorelines, particularly S5, appear to correlate between the northern and southern hemispheres on the basis of age, succession and general depth of occurrence, suggesting (1) that they may be global features controlled by the post‐glacial pattern of ice‐sheet decay; and (2) that hydro‐isostatic adjustment may exert only a minor control on the depth of particular shorelines, at least during the earlier parts of the post‐glacial transgression.
The Marl Slate: a model for the precipitation of calcite, dolomite and sulphides in a newly formed anoxic sea
Authors: SWEENEY M. , TURNER P. , VAUGHAN D. J. 
Published: June 2006

ABSTRACTDetailed studies of a new, complete Marl Slate core in South Yorkshire have provided information on isotopic (δ13C, δ18O, δ34S) and geochemical variations (trace elements and C/S ratio) which enable the formulation of a model for carbonate and sulphide precipitation in the Late Permian Zechstein Sea. Calcite and dolomite are intimately associated; the fine lamination, organic character and absence of benthos in the sediments are indicative of anoxic conditions. Lithologically the core can be divided into a lower, predominantly sapropelic Marl Slate (2 m) and an upper Transition Zone (0·65 m) of alternating sapropel and calcite‐rich and dolomite‐rich carbonates.C/S ratios are 2·22 for the Marl Slate and 1·72 for the Transition Zone respectively, both characteristic of anoxic environments. δ18O in the carbonates shows a large and systematic variation closely mirrored by variations in calcite/dolomite ratio. The results suggest a fractionation factor equivalent to a depletion of 3·8% for 18O and 1·5% for 13C in calcite. The δ34S values of pyrite are isotopically light (mean value = ‐ 32·7%) suggesting a fractionation factor for the Marl Slate of almost 44%, typical of anoxic basins.The results are related to stratification in the early Zechstein Sea. Calcite was precipitated in oxic upper layers above the halocline. Below the oxic/anoxic boundary framboidal pyrite was precipitated, resulting in lower sulphate concentration and elevated Mg/Ca ratio (due to calcite precipitation). As a result of this, dolomite formation occurred below the oxic/anoxic interface, within the anoxic water column and in bottom sediments. Variations in calcite/dolomite ratios, and isotopic variations, are thus explained by fluctuations in the relative level of the oxic/anoxic boundary in the Zechstein Sea.
Submarine limestones in the nearshore environment off Kuwait, northern Arabian Gulf*
Authors: KHALAF F. , MILLIMAN J.D. , DRUFFEL E.M. 
Published: June 2006

ABSTRACTEven with careful petrographic and mineralogic characterization of marine limestones, intertidally and subtidally lithified rocks are often difficult to differentiate, thus hindering an accurate delineation of the diagenetic environment. Limestones from water depths of 6 to 8 m off Kuwait vary in petrographic character from oosparite and biosparite (in which the cement is entirely aragonite) to oomicrite and biomicrite (in which at least some of the cement is microcrystalline magnesian calcite). Carbon‐14 dates suggest that the oosparite may have lithified at depths shallower than at present (possibly intertidally) during a lower stand of sea‐level. In contrast the biosparite, oomicrite and biomicrite appear to be contemporaneous and to have lithified subtidally.
Glacial sedimentary processes and environmental reconstruction based on lithofacies
Authors: SHAW JOHN 
Published: June 2006

ABSTRACTGlacigenic sediments exposed in pits around Villeneuve, near Edmonton, Alberta, are subdivided into facies based on grain size, sedimentary structure, glacially‐induced deformation and faulting, and groove marks. Two diamicton facies are recognised, one of which is interpreted as a primary till, deposited directly from glacier ice, and the other as a product of mass‐movement. The diamicton facies are closely associated with current bedded facies interpreted as fluvioglacial deposits. The stratigraphic sedimentological and tectonic aspects of these fluvial deposits suggest subglacial deposition in channels and cavities. At any one place the glacier appears to have alternated between being attached to the bed, causing thrusting and sole marking, and being separated from the bed by a cavity in which fluvial and mass‐movement sediments accumulated. The net result is a highly complex and laterally variable stratigraphy produced by a single glacial advance. The correct interpretation of such sequences is essential if lithostratigraphy is to be used to establish glacial history. In addition, the interpretations presented here have implications regarding the formation of soft zones in ‘till’. They indicate that the soft zones are beds of sorted sediment redeposited by mass‐movement.
Magnetic fabric characteristics of bioturbated wave‐produced grain orientation in the Bridport‐Yeovil Sands (Lower Jurassic) of Southern England
Authors: HOUNSLOW M. W. 
Published: June 2006

ABSTRACTThere is little visible primary hydrodynamic lamination preserved in the Bridport‐Yeovil Sands as a result of intense bioturbation. Where lamination is present, it exhibits wave‐produced characteristics, although current ripple lamination is also found. The grain orientation of a variety of bioturbated and non‐bioturbated fine‐grained sandstones has been determined by measuring the magnetic susceptibility anisotropy. The magnetic fabric is of a primary style and preserves two lineation directions approximately 90° apart in azimuth. These lineation directions are interpreted as the result of grain long‐axis orientations produced by wave and current processes. The magnetic fabric is dominantly carried by a small proportion of paramagnetic minerals, thought to be largely detrital chlorite and micas. This magnetic fabric has been acquired by depositional alignment of the detrital phyllosilicates and by reorientation of the phyllosilicates during the early stages of compaction. The magnetic fabric of the intensely bioturbated sandstone is not significantly different in magnitude characteristics or in the preservation of lineation directions from that of the non‐bioturbated sandstone.