bluuebunny/crossref_metadata_2025_split huggingface dataset:
Diffusion of sediment on the lee of dune‐like bedforms: theoretical and numerical analysis
Authors: CHAKRABARTI CHINMOY , LOWE DONALD R. 
Published: June 2006

ABSTRACTTheoretical and empirical analyses of flow structure, sediment transport, and sediment size characteristics at the crest of dune‐like bedforms indicate that it is possible to describe, at least semi‐quantitatively, the diffusion and deposition of sediment on the leeside of such structures. A numerical program based on this analysis simulates the grain‐size distribution and deposition rate on the leeside of dunes for specified flow conditions and bed material. Evaluation of flow and sediment variables through the numerical simulation program shows that flow velocity, flow depth and sediment size have a strong influence on the deposition rate and texture of leeside sediment before avalanching. Sorting of the bed material, in particular, appears to exert a strong control on both the grain‐size and the deposition‐rate gradients.
Diagenetic trends in the siliceous facies of the Monterey Shale in the Santa Maria region, California
Authors: PISCIOTTO KENNETH A. 
Published: June 2006

ABSTRACTX‐ray diffraction and oxygen isotopic analyses of outcrop and subsurface samples of siliceous rocks were used to reconstruct thermal and diagenetic histories of the Miocene Monterey Shale near Santa Maria, California. Within many stratigraphic sections soft, porous diatomaceous rocks change gradationally to underlying hard and brittle chert, porcellanite, and siliceous shale; the accompanying silica mineral zones are, in descending stratigraphic order: (1) biogenic silica (opal‐A), (2) cristobalitic silica (opal‐CT), and (3) microcrystalline quartz.Boundaries between silica mineral zones and stratigraphic horizons are often discordant. Within the opal‐CT zone, the d(101)‐spacing of opal‐CT decreases in a smooth non‐linear fashion from about 4 10 Å to 4‐04 Å. In the Santa Maria Valley and Bradley oil field areas the thicknesses of the opal‐CT zones are greater and the present thermal gradients less than in the adjacent Orcutt oil field. Thin opal‐CT zones at shallow maximum burial depths apparently correlate with higher thermal gradients.Using present thermal gradients and reconstructed maximum burial depths from well data in the Santa Maria region, the ranges in temperatures for the top and base of the opal‐CT zone are 38–54 °C and 55–110 °C, respectively. The temperature difference between these two boundaries ranges from 17 to 60 °C. In comparison, temperature ranges for these two boundaries computed from oxygen isotopic compositions of opal‐CT and quartz, extrapolated experimental quartz‐water fractionations, and assuming δO18= 0%o for the isotopic composition of the equilibrating fluid are 18–56 °C and 31–80 °C for the top and base of the opal‐CT zone, respectively. The temperature difference between these boundaries is 11–36 °C using this method.Thermal gradients and sedimentation rates strongly influence rates of silica transformations. Reconstructed thermal and diagenetic histories of siliceous rocks of the Monterey Shale at four well sites in the Santa Maria region demonstrate that most silica conversions probably occurred during the last 3–4 Myr in response to accelerated rates of sedimentation (and therefore burial heating) during the Pliocene.
Determination of recent deposition rates in Lake Constance with radioisotopic methods
Authors: DOMINIK JANUSZ , MANGINI AUGUSTO , MÜLLER GERMAN 
Published: June 2006

ABSTRACTSedimentation rates were determined with the 210Pb method in eight sediment cores from Lake Constance. The rate of deposition in the main basin (Obersee) varies from about 0.06 g cm−2 y−1 in the central part to 0.13 g cm−2 y−1 in the eastern part of the lake and then increases rapidly towards the Rhine delta.In the central lake area the rate of deposition has been approximately constant since 1900, and dating with the 210Pb method is in good agreement with sedimentological observations. In the Konstanzer Trichter area, the deposition rate has been increasing since about 1955 as a result of eutrophication and subsequent high carbonate production.Dating with 137Cs is fairly accurate for sediments deposited at a high rate, but is questionable for slowly accumulating ones.A positive correlation of 210Pb fluxes and sedimentation rates indicates that 210Pb flux into sediments follows the distribution pattern of solids. 210Pb profiles in four sediment cores interpreted in terms of a constant flux model display synchronous fluctuations of the sedimentation rate; however, their relation to long‐range particulate input variations remains to be proved.Sedimentation rates determined with the 210Pb method were used to calculate recent nutrient and heavy metal fluxes. Anthropogenic fluxes of Zn and Pb are in the same range of magnitude as in other polluted areas in Europe and America.
Flume experiments on the origin of flaser bedding
Authors: HAWLEY NATHAN 
Published: June 2006

ABSTRACTA series of flume runs using a 12·5 h tidal cycle was conducted in order to determine whether or not flaser beds could be formed in nearshore subtidal areas as a result of purely tidal activity. The effects of clay composition, sand size, bed thickness, and maximum current velocity were investigated. The resistance to erosion of mud beds 10 mm thick or less is low; shear stresses approximately equal to those needed to initiate sand movement resulted in rapid erosion of the mud beds. In the light of the low resistance to erosion of the beds, and the necessity for increased thicknesses and very long time periods for their resistance to be increased substantially, it is concluded that the formation of these beds is unlikely to result from purely tidal action unless very large amounts of mud are available. A mechanism which more readily permits the deposition of thicker beds and longer consolidation times, such as storm action, is preferred.
Pyrite in ammonite‐bearing shales from the Jurassic of England and Germany
Authors: HUDSON J. D. 
Published: June 2006

ABSTRACTPyrite occurs both in normal clays and shales with a benthic fauna (Oxford Clay, England, and Lias ε, Germany) and in highly bituminous shales (Lias ε, Germany). In normal shales it is present in small quantities as early framboids, but more conspicuously as internal moulds of fossils, especially ammonites. The pyrite in these is petrographically varied; several types of internal sediments and chamber linings are described and illustrated by reflected‐light and scanning electron microscopy. Most striking are pyrite stalactites, suspended from the roofs of ammonite chambers, which were later filled by calcite or baryte. Pyrite formed in reducing micro‐environments, while the sediment generally was not wholly anoxic. Most pyrite pre‐dates compaction of sediment, breakage of fossils and solution of shell aragonite. Variable rates and conditions of reduction of sea water sulphate are reflected in δ34S values ranging from −55 to +44. Stalactites probably started to form when the ammonite chambers were partially gas‐filled.In the bituminous Lias ε shales pyrite occurs abundantly as early framboids and micro‐nodules. Larger nodules show a variety of forms, some of which post‐date compaction of the sediment. Pyrite is not associated with the abundant flattened ammonites. δ34S values in shales are grouped about a mode near −20. Pyrite formed over a long time‐span, and throughout the sediment, not just in protected cavities.Contrasts in pyrite types can be related to differing depositional environments and organic contents of the shales. Pyrite is an important mineral in diagenetic mineral parageneses which can be deduced by studying fossil void‐fillings and concretions, and which help define the diagenetic history of a shale.
Surface textures of turbidite sand grains, Laurentian Fan and Sohm Abyssal Plain
Authors: WANG YING , PIPER DAVID J. W. , VILKS GUSTAVS 
Published: June 2006

ABSTRACTSurface textures of quartz grains have been examined from five samples from the Laurentian Fan and Sohm Abyssal Plain, representing varied transport distances and power of the depositing turbidity current. The grains retain their primary irregular shape derived from glacial erosion, and glacial surface textures are preserved in dish‐shaped depressions. These features have been superimposed by a slight rounding of edges and an abundance of collision‐induced markings, particularly mechanical V‐forms. The most intense current modification of this sort occurs in mid‐Wisconsinan or earlier sands that have been transported over 1000 km to the distal Sohm Abyssal Plain by turbidity currents. Collision textures probably develop during grain flow on the steep continental slope: delicate resedimented shelf foraminifera are preserved in the same turbidites and most have been transported exclusively in suspension.
The superficial sediments of the English Channel and its Western Approaches
Authors: LARSONNEUR CLAUDE , BOUYSSE PHILIPPE , AUFFRET JEAN‐PAUL 
Published: June 2006

ABSTRACTThe English Channel and its Western Approaches constitute a 700 km long epicontinental sea located in a temperate environment and in a tectonic setting where subsidence is minimal.A sedimentological survey of this region reveals quite distinct provinces: a central sector (Western Channel) where predominantly bioclastic sediments are widely represented, bounded by two mainly terrigenous‐rich zones, the outer terrace of the Celtic Sea and the Eastern Channel. Lateral sedimentary variations of decreasing grain size are interpreted in terms of current velocity patterns. The various types of such sequences may relate to the degree of mixing of older lithoclastic sediments with the Holocene bioclastic supply.A scenario for the evolution of the recent sedimentation of this epicontinental sea is presented. Starting from a permanent marine zone—at least since the Würm period—that bounds the Bay of Biscay, the Holocene transgression progressed over the English Channel. In the Western Channel earlier terrigenous deposits were gradually overlapped by bioclastic sediments that originated on the pre‐Mesozoic rocky substratum and which were particularly extensive off the Armorican Massif. In the subsequently submerged Eastern Channel the pre‐Holocene clastic source has undergone comparatively less modification and still crops out in most of the area. With the opening of the Straits of Dover, at about 9000 yr BP, submersion was complete and new hydrodynamic conditions developed as the eustatic level stabilized. The present sediment distribution is in harmony with the hydrodynamic setting except on the southern rise of the Celtic Sea where both morphology and sedimentary patterns still largely reflect pre‐Holocene conditions.
Size–density sorting of sand‐size spheres during deposition from bedload transport and implications concerning hydraulic equivalence
Authors: STEIDTMANN JAMES R. 
Published: June 2006

ABSTRACTIt is well known that sediment sorting according to size, shape and density occurs, but the exact mechanisms involved are poorly understood. To assess the effects of size and density, sand‐size spheres of two densities were transported and deposited under controlled flume conditions. Observations on the motion of discrete particles show that grains smaller than bed‐roughness grains move continuously and have the same transport velocities regardless of density. For grains near and slightly larger than the roughness, movement is intermittent and, for a given size, heavy particles move more slowly than lights. For grains much larger than bed roughness grains, movement is continuous over the rough surface and light and heavy grains have nearly the same transport velocities.Analyses of bulk sediment deposited from plane‐bed transport, show that the size and proportion of heavies decreases and that of lights increases with distance transported. For ripple bed transport, however, the size relations between associated light and heavy grains remains essentially unchanged with transport distance and the proportion of light and heavy grains is extremely variable.These results suggest that size‐density sorting in plane‐bed transport is a function of the transportabilities identified in the discrete grain studies but that sorting in ripple‐bed transport is related to deposition on, and recycling through, the bed forms. Application of these findings to the concept of hydraulic equivalence implies that some indication of bed configuration may be necessary for the concept to be useful.
Burrows and trails produced by Quinqueloculina impressa Reuss, a benthic foraminifer, in fine‐grained sediment
Authors: SEVERIN KENNETH P. , CULVER STEPHEN J. , BLANPIED CHRISTIAN 
Published: June 2006

ABSTRACTThe benthic foraminifer Quinqueloculina impressa Reuss, was buried in various types of sediment in order to assess its capability for producing sediment disturbances and thus, ichnofossils. Silts and silty‐clays showed distinct burrows; fine sands did not. Two types of burrows were produced: fairly straight, vertical burrows from 4 cm below the water‐sediment interface to within 1 cm of the sediment surface, and a horizontal and vertical maze‐like burrow system in the top centimetre of the sediment. Individuals moving on the sediment surface also produced visible trails. When the sediments were dried the burrows were always destroyed; in some cases the surface trails were preserved. We propose that the vertical burrows are escape structures and that the horizontal and vertical maze‐like structures are living burrows.Ichnofossils similar to the escape structures and surface structures have been described. Presence of these ichnofossils indicates a low energy sedimentary environment and a lack of macrofaunal bioturbation. The presence of escape structures may indicate a pulsatory pattern of sedimentation.
Impressions of algal mats from the Middle Proterozoic Belt Supergroup, northwestern Montana, U.S.A.
Authors: HORODYSKI ROBERT J. 
Published: June 2006

ABSTRACTIrregular, low relief bedding‐plane markings which probably represent impressions of thin, wrinkled algal mats have recently been detected in mudstones from the 1100 m.y. old Snowslip Formation, Belt Supergroup, Glacier National Park, Montana. Algal‐mat impressions are rarely reported from ancient strata, yet algal mats are common in certain modern environments. Originating primarily in moist settings and in relatively quiescent shallow‐water settings, such structures may be of some value in interpreting ancient depositional environments.
A study of the dynamics of a marine sandwave
Authors: LANGHORNE D. N. 
Published: June 2006

ABSTRACTThe movement of the crest of a sandwave was studied using cross‐sectional profiles obtained from lines of sea‐bed reference stakes. Measurements were made, over a six month period, before and after flood and ebb tides in relation to both spring and neap tides and surface wave conditions. Additional observations were obtained on a daily basis, over an equinoctial neap to spring to neap tidal period, in conjunction with boundary layer flow measurements. Tracer experiments were conducted to study the dispersion of sediment from the sandwave crest.The results showed that the sandwave was relatively stable at neap tides, whilst at higher tidal ranges, the crest position oscillated with successive flood and ebb tides. Net flank erosion occurred on the less steep, upstream slope during the dominant ebb tide. This, together with increased deposition on the lee slope, caused the crest to advance. It was not possible to extrapolate sandwave migration over long periods as the tidal dynamic trends were interrupted by wind stress and surface wave activity. High particle orbital velocities, generated at the sea‐bed by storm waves, caused major reductions in crestal heights.Calculated volumes of sediment eroded and accreted were used, with boundary layer flow measurements, to calculate threshold velocities for the movement of the sediment and sediment transport rates.
Quantitative mineralogical analysis of carbonate sediments by X‐ray diffraction: a new, automatic method for sediments with low carbonate content
Authors: ROSELLE PHILIPPE 
Published: June 2006

ABSTRACTA new X‐ray diffraction method has been developed whereby the weight percentages of aragonite and low and high‐magnesium calcite are determined from the integrated peak areas of samples. Peak areas are measured by a step scanning method. The weight percentages of MgCO3 in calcite are determined from the angular position of the calcite peak. This technique uses a direct calculation method which simplifies the preparation of the samples and the calibration processes and increases the quality of the results. The fully automatic method uses a desk‐top computer to guide the diffractometer and to carry out the necessary calculations. Tests on precision and accuracy of the method indicate that results with less than ± 4% error (mineral %) and ± 0.6% error (MgCO3%) are obtainable for all samples even those with a low (10%) carbonate content.
Aeolian stratification and facies association in desert sediments, Arran basin (Permian), Scotland
Authors: CLEMMENSEN LARS B. , ABRAHAMSEN KJELL 
Published: June 2006

ABSTRACT Permian aeolian sediments on the island of Arran are divisible into dune (including draa) and interdune deposits. Both types display a distinctive and unusually wide variation in grain size. The dominant features of the dune deposits are grainfall lamination, sandflow lamination, and inverse graded lamination associated with ripple‐form lamination and normal graded lamination. The flat‐lying aeolian interdune deposits are characterised by granule and sand ripples, horizontal lamination in coarse sand and granules, plane bed lamination and inverse graded lamination. Associated structures include ripple‐form lamination and deflation lags. Three types of trace fossil associated with completely bioturbated horizons occur in some low‐angle dune and interdune deposits.The aeolian facies interfinger with alluvial fan deposits giving rise to three recognizable facies belts. Marginal aeolian deposits are associated with fluvial conglomerates and are dominated by interdune deposits and occasionally very thin barchan deposits (set height 3‐37 cm). Intermediate aeolian deposits are characterized by interbedded crescentic dune, small draa (dune set height 5 cm‐4.5 m) and interdune deposits, and rare fluvial and lake sediments. Basinal aeolian deposits are dominated by draa deposits (dune set height 0.2‐28 m) associated with rare interdune sediments. Transverse dunes and draas were moved by north‐eastern palaeowinds towards the foot of the alluvial fans. The aeolian sediments were deposited in a fault‐bounded desert basin.
Quantitative interpretation of an evolving ancient river system
Authors: BRIDGE JOHN S. , DIEMER JOHN A. 
Published: June 2006

ABSTRACTMultistorey sandstone bodies described from the Upper Devonian‐Lower Carboniferous of Kerry Head (Ireland) are interpreted as deposits of aggrading, perennial, river channels migrating laterally across alluvial plains. Point bars displayed surface features such as scroll bars, chute channels and chute bars. Relatively uncommon channel fills are both coarse‐ and fine‐grained.Quantitative interpretation of the sandstone bodies was accomplished by comparison with a physical model that predicts the sedimentology of single point bar deposits developed in channels of prescribed geometry and hydraulics. This analysis reveals that the separate storeys (point bars) in each sandstone body were deposited in a single channel belt in which channel geometry and hydraulics varied little with time (order of 103 yr) and space (order of 103 m). Two southerly flowing rivers of markedly different size were responsible for all sandstone bodies: bankfull widths, depths and mean velocities of both rivers varied little with time (order of 105 yr), implying a stable climatic setting. Channel sinuosities were usually 1.15–1.2 throughout the succession. Both rivers decreased in mean channel slope as time progressed, in association with a rising base‐level and a shoreline encroaching from the south.Using Bridge & Leeder's (1979) alluvial stratigraphy model, the nature and distribution of channel sandstone bodies relative to overbank deposits in the succession can be explained by an average (compacted) floodplain deposition rate of about 0.005 m yr−1, if avulsion occurred with a frequency of about once every 103 yr. Local variation in the relative amount of channel sandstone in the succession is probably due to local tectonic control of deposition.
Transport by wind of granular materials of different grain shapes and densities
Authors: WILLETTS BRIAN 
Published: June 2006

ABSTRACTAeolian transport rates were measured for three sands: a quartz sand (relative density 2.68, sphericity 0.73), a shelly sand (relative density 2.64, sphericity 0.59, carbonate content 67%), and granular aluminium oxide (relative density 3.95, sphericity 0.67). Low sphericity depresses the transport rate, particularly at high wind speeds; high density also does so but the effect is more marked at low transporting wind speeds.The sand of low sphericity undergoes longer saltations than the other materials, but is dislodged less frequently than they are and is transported less freely in a given wind. Of the more spherical materials, the denser was the least effective at extracting energy from the wind for grain transport in the range of our experiments. Both of the more spherical materials showed evidence of a fairly sudden transition of transport behaviour at modest wind speed. It is inferred that this marks the transfer of the function of grain dislodgement from direct wind action to inter‐granular collision. The transition did not occur in experiments on the material of lower sphericity.For a given transport rate, wind speed near the bed is highest for the grains of low sphericity (by a considerable margin) and lowest for the more compact quartz grains. Propensity for transport by wind is greatest for the quartz sand, less so for the heavy material and least for the material of low sphericity.
Skeletal halites from the Jurassic of Massachusetts, and their significance
Authors: PARNELL J. 
Published: June 2006

ABSTRACTPseudomorphs after skeletal halite occur in a bed of Lower Jurassic lacustrine shale at Holyoke, Massachusetts. The pseudomorphs include pagoda types similar to those described by Southgate and show dissolution furrows. However the shales were deposited in a permanently subaqueous rather than an emergent environment, and the pseudomorphs accordingly grew in brine‐saturated sediment. Evaporation to dryness is not therefore essential to the genesis of skeletal halite.