bluuebunny/crossref_metadata_2025 huggingface dataset:
Origin of siltstones: glacial grinding versus weathering
Authors: NAHON D. , TROMPETTE R.
Published: June 2006

Siltstones are unusual rocks. They are mainly made up of quartz. Glacial grinding is considered by some authors to be the main, or even the only, process generating silt. This is supported by: (1) the presence of silt in basal tills from present ice‐caps and, above all, (2) by the volumetric importance of Quaternary loess. However, ancient glaciogenic sediments contain only a little silt scattered in the matrix of tillites, or forming thin intercalations in pro‐ and periglacial deposits. These siltstones represent less than 5% of the total volume of Upper Proterozoic and Upper Ordovician glaciogenic sequences of West Africa.

On the other hand, silt is abundant in weathering profiles of tropical, equatorial and mediterranean zones. This silt is generated by splitting of minerals, mainly quartz, inherited from the parent rock. Quartz grains are partly dissolved in situ and their fragments cemented by iron oxi‐hydroxides in tropical and equatorial zones and by calcite in mediterranean zones. Silt is particularly abundant in tropical soils, comprising up to 50‐75%. Secondary dissolution of the iron or calcite matrix induces disaggregation of these soils and the release of silt which is later reworked and concentrated by wind or running water.

Weathering processes, and especially those operating in tropical zones, are the main phenomena generating silt. Glacial grinding would generate only a little silt. A large part of the material of Quaternary loess may be derived from glacially reworked weathering profiles.
Bed‐thickness and grain size of turbidites
Authors: SADLER PETER M.
Published: June 2006

In measured sequences of limestone‐ and greywacke‐turbidites the bed‐thickness is found to vary proportionally with the fall velocity of the maximum grain size, found at the base of the bed. A simple theoretical model, based on the decay of isotropic turbulence, suggests that bed‐thickness should be a function not only of this fall velocity, but also of bottom slope, flow depth and the concentration and grain‐size distribution of the sediment in the turbidity current. The field data do show some influence of these additional factors. Nevertheless, for many natural sequences of turbidites the flows must have carried very poorly sorted sediments and the inferred flow volumes and densities must cluster tightly about modal values. Thus, grain size remains the primary variable and the modal regression curve of bed‐thickness on maximum grain size is well defined and resembles a fall‐velocity curve. Relatively steep basin floors near to source can, theoretically, cause these modal regressions for distal and very proximal parts of a turbidite to diverge, introducing a crudely parabolic appearance in the form of the total regression curve. The form of this parabolic curve predicts the deposition of thin but relatively coarse proximal beds. Such beds do occur. They are different from the thin, but relatively fine, proximal beds that have been interpreted as the result of a fractionation of a turbidity current during levee‐forming processes.
Bed‐material size and the riffle‐pool sequence
Authors: MILNE J. A.
Published: June 2006

Relationships between the bed‐material size distribution and riffle‐pool geometry of a small coarse‐bedload upland stream channel are described. Downstream sequences of bed‐material size are highly variable, although series analysis reveals some persistence in the data over short distances. Cross‐correlation of bed‐material series with bed‐elevation series indicates riffles tend to coincide with coarser bed material sequences.

Comparison of the bed‐material size distributions of individual riffles, pools and bars is, however, more revealing. Mean particle size was found to be the most useful criterion for distinguishing between bed forms, and in these terms bars are the most distinctive features. Moreover, riffles and pools can be viewed as simply downstream oscillations in elevation of a single coarser‐grained sedimentary unit. Coarse sediment supply from erosion scars created where the channel meets confining bluffs in valley‐fill deposits does mean that the average particle size of adjacent and downstream bed forms is increased. However, conclusions remained unchanged after reanalysis following the identification and subtraction of immobile particle diameters.

Plan geometry and coarse sediment supply in the upland environment are also shown to influence bed‐material size distributions. Tightly curved pools tend to have the finest pool sediments but also the least difference between bar and pool sediments. Size of material in pools is also negatively related to an index of cross‐section asymmetry.
A new mechanism for seaward dispersion of midshelf sediments
Authors: LARSEN LAWRENCE H.
Published: June 2006

Silt‐ to sand‐size particles may be transported across middle continental shelves by currents forced by the radiation stress of variable‐amplitude swell. With the passage of each group of waves of sufficient amplitude for the high waves in the group to resuspend bottom sediments, there is a seaward bias in the return of sediments to the seafloor. Over a storm season, with the passage of many wave groups, the net seaward sediment displacement may be 1‐5 km.
Burial diagenesis and crystal diminution. The origin of crystal diminution in some limestones from South Wales
Authors: DIXON JOHN , WRIGHT V. PAUL
Published: June 2006

ABSTRACTCarboniferous speleothem calcites in un‐metamorphosed limestones from South Wales exhibit crystal diminution (degrading neomorphism) textures. These textures result from processes which have been considered only important under metamorphic conditions. The textures are interpreted as the result of recrystallization associated with dislocation processes. These processes relate to low temperature creep associated with low strain rates extended over long periods of time. The textures described here occur in unusually large crystals but similar textures have been described in limestones from deep boreholes. Dislocation processes, recovery and recrystallization are likely to be important processes operating during deep burial diagenesis.
Practical representation of characteristic grain shape of sands: a comparison of methods
Authors: WILLETTS B. B. , RICE M. A.
Published: June 2006

ABSTRACT A measure of grain shape is needed for incorporation in calculations of the behaviour of grain populations (for example during transport by fluids). Many shape measures have been proposed, most of them for application to single grains rather than to populations. In this paper three such shape parameters are evaluated for samples taken by size fraction from each of three parent sands. The chosen parameters are the maximum projection sphericity of Sneed & Folk (based on triaxial measurements made on the grains), the dynamic shape factor of Briggs (based on settling velocity in water), and rollability, after Winkelmolen (based on rolling behaviour in a specially mounted rotating cylinder).

It is shown that the Sneed & Folk parameter and rollability both discriminate clearly between the shape characteristics of the three sands over the size range 150‐500 μm. Moreover the discrimination of the two parameters is mutually consistent. However, dynamic shape factor gives results which for sizes smaller than 300 μm are inconsistent with those of the other two methods and which do not discriminate reliably between the populations. This is inevitable because the differences between drag on spheres and on other shapes become very small at Reynolds Numbers corresponding to those which obtain in settling tests on grains smaller than 300 μm.
Size—frequency distribution of longitudinal dune rippled flank sands compared to that of slipface sands of various dune types
Authors: SNEH AMIHAI , WEISSBROD TUVIA
Published: June 2006

Rippled flanks of longitudinal dunes and the slip faces of various dune types can be distinguished granulometrically by comparing the top to bottom trend of changes of sorting in the sand population of each individual slope. Flank sands, which are prone to sorting processes through migration of ripples alongslope, are always better sorted upslope whereas slipface sands, which are controlled by avalanche and grainfall of sands, become better sorted downslope. Considering the absolute values of the bottom samples, the sands of the slip faces are both better sorted and the size distribution more symmetrical than those of the rippled flanks.

Applying this approach in investigating ancient sediments could improve palaeogeographical interpretations.
Progradation geometries of carbonate platforms: examples from the Triassic of the Dolomites, northern Italy
Authors: BOSELLINI ALFONSO
Published: June 2006

Various types of progradation of Triassic carbonate platforms are described from the Dolomites of the Southern Alps. The internal and external geometric relationships are exposed in spectacular natural sections and, moreover, their scale (500–1000 m of thickness) is such that they can be compared with features found in seismic profiles.

The different types of progradation are controlled by a number of factors which, normally, interact with each other. These factors include: rate of basinal sedimentation, rate of subsidence, width of the platform, depth of the surrounding basin and eustatic variations of sea‐level.

Progradation is not a continuous process but episodic. Moments of massive debris input, during which the platform advances, alternate with long periods of negligible progradation, during which basinal sediments accrete and onlap the toe of slope. Upper boundary relationships of the prograding platforms include offlap, toplap and erosional truncation. Lower boundary relationships are horizontal, climbing and descending progradations. A variety of phenomena and circumstances have caused the cessation of progradation of the Triassic platforms. They include volcanism, collapse of margins, drowning (rapid relative rise of sea‐level), subaerial exposure (relative fall of sea‐level) and, probably, a natural decay of the system.

In the Triassic of the Dolomites, two main progradation models can be put forward: in the Ladinian model, progradation took place simultaneously with aggradation (relative rise of sea‐level), whereas the characteristic feature of the Carnian model is toplap (relative stillstand of sea‐level).
Upper Silurian lithistid sponge reefs on Somerset Island, Arctic Canada
Authors: NARBONNE GUY M. , DIXON OWEN A.
Published: June 2006

The Upper Ludlow Douro Formation contains the first reported Silurian sponge reefs. These relatively small (5–35 m diameter), mound‐shaped structures contain, on average, 35% lithistid demosponges. Reefs are surrounded by irregular haloes of crinoid debris; abundance and diversity of all fossil groups decreases away from the reefs. Each reef is underlain by a lens of crinoid wackestone to grainstone rich in crinoid holdfasts; trepostomate bryozoans, solenoporacean algae and rhynchonellid brachiopods are locally common. The bulk of each reef consists of lime mudstone with abundant lithistid sponges. This is capped by a thin layer of wackestone with abundant tabulate and rugose corals and fewer lithistid sponges, calcareous algae, trepostomate bryozoans and stromatoporoids. This zonation, in which a sponge colonization community was replaced by a coral diversification community, is similar to that reported from some Middle Ordovician, Upper Jurassic and Holocene sponge reefs.

The Douro sponge reefs were relatively low structures, with about 3 m maximum topographic relief. They grew on a broad carbonate platform, probably in warm, tranquil, turbid waters of normal or near‐normal marine salinity. Periodic influxes of terrigenous mud adversely affected reef size, and caused biotic changes. Some of the reef lime mud was derived from non‐reef sources, but significant quantities were also produced on the reefs. Reefs underwent synsedimentary lithification, bioerosion and minor storm erosion. Fabrics and compositions of sparry calcite in cavities record three generations of meteoric cementation. Originally siliceous spicules of the lithistid sponges were dissolved and the moulds later filled with sparry calcite. Early dissolution of siliceous spicules is common in reef environments, and may have caused fossil sponges to be under‐represented in ancient reefs.
Characteristic features of pockmarks on the North Sea Floor and Scotian Shelf
Authors: HOVLAND MARTIN , JUDD ALAN G. , KING LEWIS H.
Published: June 2006

This paper presents the results of a comparative study of pockmarks and associated features appearing on both sides of the North Atlantic: on the Scotian Shelf off Nova Scotia and in the northern North Sea. Pockmarks are formed in seabed material consisting of soft silty clay. The seismic, sonar and lithologic characteristics of the sediments on the Scotian Shelf are remarkably similar to those found in the northern North Sea. Sediment clouds suspended in the water column immediately over the seabed have previously been observed on side‐scan records associated with gas‐charged sediments on corresponding shallow‐seismic records. These and similar observations strongly suggest that most pockmarks are caused by gas efflux through the seafloor. However, the detailed mechanism of formation and the origin of the gas in the sediments is still unknown.
Pyritization of tubes and burrows from Late Pleistocene continental shelf sediments off North Norway
Authors: THOMSEN ELSEBETH , VORREN TORE O.
Published: June 2006

Four different types of pyritized tubes and three types of pyritized burrow fillings are described from Pleistocene glaciomarine sediments in Andfjorden, northern Norway. The tubes and burrows probably originated from tubicolous and burrowing polychaetes respectively. The decomposition of the organic matter in the tubes and burrows created a reducing micro‐environment favouring precipitation of pyrite. By comparison with Holocene tubes from marine sediments in Andfjorden and FugloSyfjorden, it is seen that pyritization commenced with isolated spherules. These spherules with incipient pyrite crystals and framboids were formed mainly on the inner wall of the tube. Presence of a monosulphide in the Holocene Fugløyfjorden material suggests that the pyritization process has reached a later phase; the final result would be a completely pyritized trace fossil. It is shown that single pyrite crystals (octahedra) generally attain greater size in the burrow fillings than in the tubes. The microstructure found in some of the pyritized tubes is interpreted as a reflection of the microstructure in the original wall. Finally, the implications for the depositional environment in Pleistocene in Andfjorden is investigated with reference to the benthic skeletal macrofaunal assemblage in the sequence. The pyritized trace fossils occur frequently in an opportunistic assemblage from a period (c. 14,000–13,000 yr BP) characterized by some oxygen deficiency. Later (13,000–10,000 yr BP) they play a minor role in an established assemblage under improved oxygen conditions.
Composition of plagioclase feldspar in modern deep‐sea sands: relationship to tectonic setting
Authors: MAYNARD J. BARRY
Published: June 2006

Microprobe analysis of plagioclase in deep‐sea sands from various tectonic settings gives averages ranging from 19% An for trailing‐edge continental margins to 56% An for basins on the ocean side of an island arc. For each setting except island arcs, anomalously high amounts of pure albite are found. These albitic grains may be recycled from pre‐existing sedimentary rocks. Potassium‐rich plagioclase, thought to be indicative of a volcanic provenance, is less abundant than would be expected from the proportion of volcanic rock fragments. Perhaps such high‐K plagioclases are less resistant to weathering than low‐K plagioclase.
Holocene reef sequences and geochemistry, Britomart Reef, central Great Barrier Reef, Australia
Authors: JOHNSON DAVID , CUFF CHRISTOPHER , RHODES EUGENE
Published: June 2006

Holocene reef development was investigated by coring on Britomart Reef, a mid‐shelf reef, 23 km long and 8 km wide situated 120 km north of Townsville in the central Great Barrier Reef (GBR). Two holes were drilled, Britomart 1 on a lagoon patch reef, and Britomart 2 on the windward reef crest. The Holocene reef (25·5 m) is the thickest yet recorded in the GBR and overlies an uneven substrate of weathered Pleistocene limestone.

Mineralogical and geochemical analyses show that magnesian calcite and aragonite were converted to low Mg‐calcite below the Holocene‐Pleistocene disconformity. Corals above the interface have 7500–8500 ppm Sr, but 1650–1500 ppm just below it, decreasing to 400–800 ppm downwards. The intermediate Sr values could be due to partial replacement of aragonite by calcite or higher original Sr content in the corals.

Three units are recognized in the Holocene: (1) coral boundstone unit, (2) coral framestone unit, and (3) coral rudstone unit. The coral boundstone unit forms the top 5 m of both cores and is algal‐bound coral rubble similar to the present reef top. The coral framestone unit is composed of massive head corals Diploastrea heliopora and Porites sp., and is currently forming in patch reefs situated in the lagoon and along the reef front. The coral rudstone unit comprises coral rudstone and floatstone with unabraded, and unbound, coral clasts in muddy matrix. This matrix may be up to 30% sponge chips.

Radiocarbon dating indicates the reef grew more rapidly under the lagoon than under the reef front from 7000 to 5000 yr BP. The rate of reef growth matched existing estimates of sea‐level rise, but lagged approximately 1000 years (5–10 m) behind it. Most of the reef mass accumulated between 8500 and 5000 yr BP as a mound of debris, perhaps stabilized by seagrasses or algae. Accretion of the reef top in a windward direction between 5000 and 3000 yr BP created the present, steep reef‐front profile.
Modelling of turbidity currents on Navy Submarine Fan, California Continental Borderland
Authors: BOWEN ANTHONY J. , NORMARK WILLIAM R. , PIPER DAVID J. W.
Published: June 2006

Several Holocene turbidites can be correlated across much of Navy Fan through more than 100 sediment core localities. The uppermost muddy turbidite unit is mapped throughout the northern half of the fan; its volume, grain‐size distribution and the maximum height of deposition on the basin slopes are known. These parameters can be related to the precise channel morphology and mesotopography revealed by deep‐tow surveys. Thus there is sufficient information to estimate detailed flow characteristics for this turbidity current as it moved from fan valley to distal basin plain.

On the upper fan, the gradient and the increasing downstream width of the channel and only limited flow overspill suggest that the flow had a Froude number close to 1.0. The sediment associated with the channel indicates friction velocities of about 0.06 m s−1 and flow velocities of about 0.75 m s−1. Using this flow velocity and channel dimensions, sediment concentration (∼2×10−3) and discharge are estimated, and from a knowledge of the total volume of sediment deposited, the flow duration is estimated to be from 2 to 9 days. It is shown that the estimates of Froude number, drag coefficient, and sediment concentration are not likely to vary by more than a factor of 2.

On the mid‐fan, the flow was much thicker than the height of the surface relief of the fan and it spread rapidly. The cross‐flow slope, determined from the horizontal extent of turbidite sediment, is used to estimate flow velocity, which is confirmed by consideration of both sediment grain size and rate of deposition. This again allows sediment concentration and discharge to be estimated. The requirements of flow continuity, entrainment of water during flow expansion, and observed sediment deposition provide checks on all these estimates, and provide an integrated picture of the evolution of the flow. The flow characteristics of this muddy turbidity current are well constrained compared to those for more sand‐rich late Pleistocene and early Holocene turbidity currents on the fan.
Shallow subsurface diagenesis of Pleistocene periplatform ooze: northern Bahamas
Authors: MULLINS HENRY T. , WISE SHERWOOD W. , GARDULSKI ANNE F. , HINCHEY EDWARD J. , MASTERS PHILIP M. , SIEGEL D.I.
Published: June 2006

Previous studies on early submarine diagenesis of periplatform carbonates have implied that these originally polymineralic (aragonite, magnesian calcite, calcite) sediments are susceptible to early diagenesis only in current‐swept open seaways or where surficially exposed by erosion on the seafloor. It has also been proposed that while in the shallow subsurface, periplatform oozes retain their original mineralogy for at least 200,000–400,000 yr and remain unlithified for tens of millions of years.

Evidence is reported here for extensive calcitization and selective lithification of periplatform oozes of late Pleistocene age in two piston cores collected from water depths of ∼ 1,000 m north of Little Bahama Bank. It is shown that shallow (<30 m) subsurface diagenesis can significantly alter the original mineralogy of periplatform oozes to predominantly calcite in less than 440,000 yr, and that cementation by calcite can produce chalk‐ooze sequences within the same time‐frame. Periplatform oozes that originally contain a high percentage of bank‐derived magnesian calcite appear to have a higher diagenetic potential than those originally low in magnesian calcite. Shallow subsurface calcitization and fithification greatly reduce the diagenetic potential of periplatform carbonates, and chalk‐ooze sequences apparently can persist for tens of millions of years and to burial depths of at least 300 m.

Shallow subsurface diagenesis, at water depths > 1,000 m, proceeds via dissolution of magnesian calcite and aragonite and reprecipitation of calcite as allochem fillings, exterior overgrowths and cement. It is speculated that density‐driven ‘Kohout convection‘, where seawaters under‐saturated with respect to magnesian calcite and aragonite and saturated/supersaturated with respect to calcite flow through the margins of carbonate platforms, is the primary driving mechanism for shallow subsurface diagenesis. Removal of Mg during early stages of deep seafloor and shallow subsurface diagenesis should increase the Mg content of interstitial waters which is likely to increase the ‘dolomitizing potential’ of Kohout convection fluid flow.
Evidence of dynamics reversal in tropical estuaries, geomorphological and sedimentological consequences (Salum and Casamance Rivers, Senegal)
Authors: BARUSSEAU J. P. , DIOP E. H. S. , SAOS J. L.
Published: June 2006

South Dakar Senegambian estuaries are subject to an unusual hydrodynamical regime caused by weak or absent run‐off. In the Salum delta, each distributary lacks fresh water during most of the year. Only the tidal flows are responsible for geomorphological and sedimentological effects. The current distribution shows a net discharge upstream due to the extensive evaporation and evapotranspiration in mangrove swamps and tidal flats. Consequently the salinity is always higher towards the river than near the sea. A high salinity bottom layer suggests the occurrence of a supersaline wedge of reverse sense to the salt wedge of a normal estuary. Such an inverse pattern is similarly displayed by sedimentological features (double upstream turned spits) and by the external location of the turbidity maximum. A coherent reverse estuary model is suggested from our field observations.
Field observations of the threshold of sediment motion by wave action
Authors: DAVIES A. G.
Published: June 2006

Two field experiments were carried out to determine critical conditions at the threshold of sand motion beneath irregular sea waves. The experiments were performed outside the breaker zone at Blackpool Sands, Start Bay, Devon, in waters of depth 4–10 m. From synchronous video and near‐bed velocity records, critical conditions at the threshold of motion have been established in terms of the measured free‐stream velocity amplitude. Despite the irregular nature of the (swell) waves and the mixture of grain sizes on the bed, agreement between the present field results and previous laboratory results is reasonable, provided that proper allowance is made for the presence of sand ripples. In order to define threshold motion conditions on a more rational basis than in terms of the free‐stream velocity amplitude, calculations of the bottom stress have been made on the basis of laboratory‐derived wave drag coefficients. The threshold‐motion conditions thus established from the field data are in good agreement with sediment threshold values from Shields' curve derived from laboratory work, at least for cases in which it has been possible to calculate the skin‐friction contribution to the total bed shear stress.
Composition and mineralogy of suspended sediment in the fluvio‐estuarine zone of the Loire River, France
Authors: MANICKAM S. , BARBAROUX L. , OTTMANN F.
Published: June 2006

The fluvio‐tidal transition of suspended sediment in terms of mineralogy and composition in the Loire River drainage basin, the largest French river basin, was investigated in the fluvial zone at Montjean and in the tidal zone at Mauves‐Thouaré, for a complete seasonal cycle. At Montjean, where the river experiences unidirectional flow, the composition and mineralogy (especially clays and clay minerals) of river suspended material (RSM) are governed by the river discharge, upstream contributions, climatic conditions and microbiological activities. However, due to reversing tidal and river currents at Thouaré, in the zone of tidal dynamics, these relations are changed. In the downstream direction sand and clay content in the RSM decrease while the silt content increases. Among clay minerals, between these two observation stations, montmorillonite remains constant, kaolinite diminishes, and the other minerals increase downstream. Combustible material (organic) and nitrate (NO3) contents in the RSM increase whereas the phosphate (PO43‐) and CaCO3 contents decline considerably during transport. At the head of the tidal zone, where the river flow encounters the tidal influence, there is a relatively stationary water mass (tidal slack) where sands, clays, phosphate, carbonate and silica are deposited by physical and chemical processes. Physical sedimentation takes place by simple gravitational deposition (sands), and by sorting and complicated differential settling (clays); chemical sedimentation takes place by precipitation (calcite‐CaCO3; apatite‐Ca5 (PO4)3 (OH,F,Cl); coagulation of dissolved silica‐SiO2) in connection with seasonal algal bloom and eutrophic events.
Asymmetry of Zoophycos burrows as a way‐up criterion—a reconsideration
Authors: WETZEL ANDREAS
Published: June 2006

The asymmetric fill of Zoophycos spreiten observed in cross‐sections cannot always be used as a reliable geopetal indicator. Rather, the use of the entire burrow system is more reliable because of the asymmetry of the whole trace fossil.
Late Quaternary sedimentation on the Southern Sohm Abyssal Plain
Authors: VILKS G. , BUCKLEY D. , KEIGWIN L.
Published: June 2006

Six closely spaced sediment cores taken below the carbonate compensation depth penetrated fine silty muds and entered sandy sediment at 10–12 m below the seafloor. Foraminiferal assemblages and δ8O analyses on planktonic foraminifera indicated that the surface muds down to 2 m are Holocene and derived from local promontories above the CCD. Below these sediments are about 6 m of clays deposited during the late Wisconsin. These are unfossiliferous and have a possible northern source suggested by the higher chlorite content. Sandy sediments below 9 m in the cores contain well preserved benthic foraminifera from the Scotian Shelf. Glacial δ18O values on planktonic tests indicate the sandy sediments are most likely of latest Wisconsin age. Thus during the recent interglacial, the sand fraction of the southern Sohm Abyssal Plain sediments is mostly locally derived, but during glacial periods the sediments have a distant northern source containing quartz sand that was initially deposited on the Scotian Shelf 1,500 km to the north.
Pivoting analyses of the selective entrainment of sediments by shape and size with application to gravel threshold
Authors: KOMAR PAUL D. , LI ZHENLIN
Published: June 2006

Measured variations of pivoting angles with grain size, shape (‘reliability’ and angularity) and imbrication are employed in analyses of grain threshold to examine how these factors influence selective grain entrainment and sorting. With a bed of uniform grain sizes, as employed experimentally to establish the standard threshold curves such as that of Shields, the threshold condition depends on grain shape and fabric. The analysis demonstrates quantitatively that there should be a series of nearly‐parallel threshold curves depending on grain pivoting angles. For a given grain size, the order of increasing flow strength required for entrainment is spheres, smooth ellipsoids (depending on their ‘reliability’), angular grains, and imbricated ellipsoids (depending on their imbrication angles). The relative threshold values for these different grain shapes and fabric are predicted according to their respective pivoting angles, but remain to be directly tested by actual threshold measurements.

The pivoting angle of a grain also depends on the ratio of its size to those it rests upon. This dependence permits an evaluation of selective entrainment by size of grains from a bed of mixed sizes, the condition generally found in natural sediments. The pivoting model predicts systematic departures from the standard threshold curves for uniform grain sizes. Such departures have been found in recent studies of gravel threshold in rivers and offshore tidal currents. The pivoting model is compared with those threshold data with reasonable agreement. However, more controlled measurements are required for a satisfactory test of the model.

It is concluded that variations in pivoting angles for grain entrainment are significant to the processes of selective sorting by grain size and shape.
Sedimentology and morphology of a low‐sinuosity river: Calamus River, Nebraska Sand Hills
Authors: BRIDGE J. S. , SMITH N. D. , TRENT F. , GABEL S. L. , BERNSTEIN P.
Published: June 2006

A study reach of the Calamus River, Nebraska Sand Hills, has a low sinuosity (less than 1.3) and braiding parameter (less than 1). Depending on sinuosity, the channel is occupied by alternate bars and point bars, the emergent parts of which form nuclei for midstream bars (islands). Channel migration occurs by bend expansion and translation, downstream and lateral growth of islands, and by chute cutoff.

Channel‐bed sediment is mainly medium‐grained sand, but gravel and coarser sand sizes occur in thalweg areas adjacent to cutbanks and upstream parts of bars and islands, and finer sands occur on the downstream parts of bars and filling channels. Curved‐crested dunes cover most of the channel bed at most flow stages, with ripples restricted to shallow areas near banks. Bed material is mostly large‐scale cross‐stratified, with small‐scale cross‐strata interbedded with plant debris occurring in topographically high areas near banks.

Vibracores through channel bars show a basal erosion surface overlain by large‐scale cross‐stratified sands, in turn overlain by small‐scale cross‐stratified sand interbedded with plant debris. The overall sequence generally fines upwards, but the large‐scale cross‐stratified portion either fines upwards, coarsens upwards, or shows little grain size variation. Lithofacies distributions vary spatially within and between bars depending on position in the bar and local channel curvature/width, in a similar way to unbraided rivers elsewhere. Lithofacies of bar deposits are similar to those in the active channel, and the elevations of the basal erosion surface and adjacent channel thalweg correspond closely.

Channels abandoned by chute cutoff are filled progressively from the upstream end, and comprise deposits similar to the downstream parts of bars (i.e. fining upwards). The downstream extremities of channel fills may contain large proportions of peat relative to sand, but little mud due to the paucity of such fine suspended load in the Calamus.
Classification and analysis of palynodebris from the Palaeocene sediments of the Forties Field
Authors: BOULTER MICHAEL C. , RIDDICK ANDREW
Published: June 2006

Sedimentary organic matter behaves as sedimentary particles and this study attempts to interpret its occurrence in the Palaeocene sands and mudstones of the Forties Field of the North Sea. New nomenclature and classification of this palynodebris are presented and are used to make quantitative counts of the palynodebris from nearly 600 samples of known sedimentary character. The new categories are further refined by objective testing of the data by Principal Components Analysis and Cluster Analysis. It is argued that two types of sedimentary organic matter are of particular value because they are preferentially deposited in different environments: palynowafers are most commonly deposited in the submarine fan lobes and channel complexes and amorphous matter occurs mostly in the lower energy basin plain sediments.
Isotope geochemistry, petrology and depositional environments of the diatomite‐dominated Tripoli Formation (Lower Messinian), Sicily
Authors: BELLANCA A. , CALDERONE S. , NERI R.
Published: June 2006

ABSTRACT The Tripoli Formation (Lower Messinian) in Sicily includes diatomites irregularly alternating with marl and carbonate beds and lies, stratigraphically, between the Tortonian pelagic marls and the evaporitic Calcare di base. The relationships between mineralogy, textural features and oxygen‐carbon isotopic compositions of carbonate components point to a wide variability of depositional conditions and suggest that Tripoli sedimentation occurred in small basins characterized by periodic and marked restriction from the open sea.

The isotopic values of calcite and dolomite in the diatomites suggest an evolution from normal marine towards more restricted environments. Evaporating conditions are also indicated by the occurrence of anhydrite, length‐slow chalcedonic quartz and moulds of gypsum. In a more advanced stage, the precipitation of heavy δ180 dolomite in the interstitial pores of fossil‐poor diatomites denotes an environment with highly evaporated water. Mixing of meteoric and marine waters, on the other hand, might have favoured the precipitation of a dolomite characterized by relatively low δ180 and δ13C values.

The deposition of marl and carbonate beds alternating with or overlying the diatomites took place in an environment with highly evaporated marine waters on the basis of δl18O values of dolomite (up to + 9.10‰) and aragonite (up to + 5.83‰), occurrence of evaporitic minerals and lack of fossils. The presence at these levels of calcite with extremely negative δ13C values (down to ‐ 38.40‰), filling gypsum moulds, suggests activity of sulphate‐reducing bacteria. Some aragonitic marls, however, bear evidence of deposition in relatively normal marine conditions.
Sedimentological effects and palaeoecological implications of the tube‐building polychaete Lanice conchilega Pallas
Authors: CAREY DREW A.
Published: June 2006

The tube‐building polychaete Lanice conchilega forms intertidal mounds in association with macroalgae. This assemblage produces structures that can record tidal and seasonal cycles in the stratigraphic record. They mark low‐tide levels because the assemblage occurs below neap low tide and many of the structures form when water drains off the assemblage.

Mounds are created by disturbance of the sediment surface around aggregations of the assemblage and accumulation of sediments within the aggregations. The initial aggregations may be relict patches of adult worms, new clumps of juveniles or both. Juvenile worms and drift algae easily settle and survive in high‐density patches, whereas predators and waves more readily disturb low‐density patches. Algae and tubes extend through accumulated sediment and create a new, higher surface. Through this interactive process the high‐density patches increase height rapidly while the low‐density patches erode. Regardless of density, when the initial distribution is regular the surface remains flat, rising evenly or eroding evenly. Accumulation and erosion increases during spring tides and decreases during neap tides.

Mound development also follows a seasonal pattern. The recruitment of juvenile worms in spring facilitates algal settlement and initiates mound buildup. The rate of tube‐building and algal growth increases in early summer as the erosive forces of storms decline. This leads to the greatest development of mounds in late summer just before the macroalgae begin to die. The loss of algae coincides with autumnal storms, and causes catastrophic erosion of the mounds into relict patches of dormant tube‐worms. Patches of tubes that survive the winter enhance spring recruitment and renewal of mound development.

The shape of the mounds is often obscured during burial and preservation. Because of this, it may be difficult to distinguish the original form of ancient mounds without contemporaneous cementing organisms. But cycles of deposition and erosion are recorded in Lanice tubes and other biogenic structures. These structures can be used in conjunction with physical structures to define tidal height, seasonality or current regime. In general however, the presence of tube aggregations per se is not diagnostic of a specific hydraulic environment.
Interference phenomena between particle flattening and particle rounding in free vertical sedimentation processes
Authors: GOOSSENS DIRK
Published: June 2006

In this paper interference phenomena between particle flattening and particle rounding during the free vertical sedimentation of natural particles are investigated. Particle flattening is measured by the Corey Shape Factor (CSF), particle rounding by the Powers Index (P). Using Dietrich's equation, the effect of the flattening and the effect of the rounding on the terminal fall velocity of the particle can be studied separately. This is done by examining the influence of particle flattening and particle rounding on the Particle Sedimentation Coefficient (PSC), which is the ratio of the nominal diameter Dn of the particle to the equivalent sedimentation diameter Db. It appears that particle flattening and particle rounding are interdependent so that interference phenomena occur. In the case of positive interference, the combined effect of flattening and rounding on the PSC is larger than the sum of the effects of both parameters separately. If, on the other hand, the combined effect is smaller than the sum of the single effects, negative interference occurs.

For fine‐grained particles, the more the shape of a particle differs from a sphere in one variable (flattening or rounding), the higher will be the influence of the other variable on the free vertical fall. For coarse‐grained particles, the inverse occurs. For very fine‐grained particles (Dn < 200 μm (air) or < 370 μm (water)) the influence of particle rounding on free vertical settling is negligibly small. For coarser grained particles, flattening is the most important parameter if the grains are strongly flattened. If not, rounding will be the governing parameter.
Quantitative X‐Ray diffraction of carbonate sediments: mineralogical analysis through fitting of Lorentzian profiles to diffraction peaks
Authors: MARTINEZ B. , PLANA F.
Published: June 2006

A new automatic method for quantitative mineralogical analysis of carbonate sediments by X‐Ray diffraction is described. Percentage analysis has been carried out by means of the reference intensities method from integrated areas of the diffraction peaks. Diffraction peaks were fitted by means of Lorentzian profiles. The addition of an internal standard allows the determination of data on the stoichiometric disequilibrium of the carbonate phases and the existence of non‐carbonate phases and/or non‐crystalline phases simultaneously. Relative errors of the quantitative estimates of carbonate phases are always less than 15%, even though real percentage differences between actual and calculated values are less than 3·8% in all cases.
The characteristics, classification and origin of late Pleistocene fan deposits in the Cass Basin, Canterbury, New Zealand
Authors: McARTHUR J. L.
Published: June 2006

The deposits comprising ‘valley‐side fans’ in a small intermontane basin of the Southern Alps are classified as debris flow, water‐laid, intermediate and mixed deposits on the basis of particle size and clast orientation characteristics. Five varieties of debris flow deposit are identified including unimodal and bimodal ‘mudflow gravels’. The fans comprise mainly unimodal mudflow gravels which although apparently similar to the mudflow gravels described from montane and periglacial environments are coarser, have less silt and clay and are better sorted than the mudflow deposits described from semiarid alluvial fans. Additions of airborne silt and fine sand to the fan catchments during later stages of fan building gave rise to bimodal debris flow deposits which appear similar to gravels described from cold‐climate fans in Tasmania and classed as water‐laid deposits. Braided stream deposits were added to the depositional sequence towards the closing stages of fan building indicating that the fan had become ‘wetter’. In many places, however, the youngest Pleistocene fan deposits are silt‐rich mixtons reflecting a peak in loess deposition.
Ultra‐fine structures and genesis of the Campanian Negev high‐grade phosphorites (southern Israel)
Authors: SOUDRY DAVID
Published: June 2006

Ultra‐fine structures of the Negev high‐grade phosphorites provide valuable clues to unravelling the genesis of these rocks, the question of their differential areal distribution, and the biosedimentary mechanisms involved in their considerable enrichment. Peculiar to these phosphorites is an intergranular phosphate matrix, for the most part constituted by a variety of phosphatic microbial tubules displaying a range of spatial micro‐organizations. The phosphate particles fixed by this filamentous meshwork also consists of internally organized and non‐organized packed microbial remains of different types.

Analogies between the fabrics of the matrix and the corpuscles lead to conception of a two‐stage depositional scheme for these phosphorites, based on rhythmical repetition of two sedimentary mini‐events, in a slightly oscillating, very shallow marine system—(1) a low‐energy event of microbial colonization of the Mishash bottoms, followed by early phosphatization of the organic structures mainly in marginal situations; and (2) a higher‐energy event which broke up the phosphatic mats into debris, redepositing them as clastic layers in nearby basinward sites, while becoming bound by a new meshwork of filamentous microphytes. The differential phosphatization of the intergranular microbial binder, again occurring mostly in marginal localities, produced highly enriched phosphorites. Minor truncations and redepositions leading to amalgamation of the successive layers account for the massive fabric now displayed by most of these rocks. Examining the structural and textural features, the validity of the sedimentary mechanisms of Recent phosphorite formation for the Campanian Negev rocks is discussed.
A numerical technique for analysing pebble‐form variations, applied in Loch Earn, Scotland
Authors: MCMANUS JOHN , ALRASOUL ALI H. A.
Published: June 2006

The lack of a suitable method of comparing the form assemblages of pebbles in naturally occurring populations was identified during studies of the shorelines of Loch Earn. In the absence of a precise intercomparison system between the verbal descriptors, a numerical method is introduced based on the Sneed & Folk (1958) grid. This permits mean, single and double pebble‐form field scores to be calculated. Average form values for ‘compactness’, ‘platiness’ and ‘elongateness’ are computed from rotations of the grid, and simple numerical values are derived to describe mean pebble populations. The technique enables some quantification of pebble‐form diversity. Each form of analysis is demonstrated using real data from lake shorelines.
Diagenetically enhanced bedding in argillaceous platform limestones: stratified cementation and selective compaction
Authors: BATHURST ROBIN G. C.
Published: June 2006

A field and petrographic study has been made of 34 massive beds in argillaceous limestones of open marine platform facies in the U.K. Lithologies include grainstones, packstones, wackestones and lime mudstones. The rocks are of Silurian, Carboniferous and Jurassic ages. Additional information was obtained from other limestones in the U.K., the U.S.A., Canada and continental Europe. The beds are parts of sequences composed of couplets of strata,fissile limestonesalternating withhard limestones.In the fissile limestones the effects of mechanical compaction and pressure‐dissolution have been concentrated, whereas in the hard limestones they are minimal or absent. Bedding planes visible in outcrop are diagenetic in origin and lie in the middle parts of the fissile limestones where compaction has been most severe. The features produced by pressure‐dissolution are dissolution seams and fitted fabric: there are no stylolites. The original carbonate sediments were bioturbated and any structures produced by flowing water were destroyed. The vertical distribution of the bedding planes bears no relation to primary depositional bedding planes which are rare or absent.

It is inferred that the strata which were to become the hard limestones were selectively cemented before mechanical compaction had been completed. Thenceforth, mechanical compaction and then pressure‐dissolution were concentrated in the less cemented strata: these became the fissile limestones. Pressure‐dissolution acted late in the diagenetic history and provided only an insignificant part, if any, of the carbonate for cementation.

It is concluded that the orientation of beds (couplets) is parallel to successive sea floors and that the sediments that eventually became single beds accumulated synchronously. Similar couplets in platform limestones of the Mississippian and Pennsylvanian of the U.S.A. extend over thousands of square kilometres.

The signal that controlled the initial selective cementation must have been widespread and synchronous and also syndepositional in its timing but otherwise cannot be further defined on the basis of the data so far collected. The presumed order of events was (1) accumulation of carbonate sediment, terrigenous clay and organic matter, (2) hydrodynamic reworking and bioturbation. the latter finally overprinting the former, (3) selective cementation of the more carbonate‐rich strata yielding couplets, each consisting of a relatively well‐cemented stratum and a poorly cemented stratum, (4) mechanical compaction concentrated in the less cemented strata, (5) pressure‐dissolution concentrated in the same strata.