bluuebunny/crossref_metadata_2025_split huggingface dataset:
Upper Silurian lithistid sponge reefs on Somerset Island, Arctic Canada
Authors: NARBONNE GUY M. , DIXON OWEN A. 
Published: June 2006

ABSTRACTThe 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

ABSTRACTThis 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

ABSTRACTFour 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

ABSTRACTMicroprobe 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

ABSTRACTHolocene 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

ABSTRACTSeveral 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

ABSTRACTPrevious 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

ABSTRACTSouth 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

ABSTRACTTwo 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

ABSTRACTThe 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

ABSTRACTThe 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

ABSTRACTSix 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

ABSTRACTMeasured 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

ABSTRACTA 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

ABSTRACTSedimentary 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.