bluuebunny/crossref_metadata_2025_split huggingface dataset:
The morphological dynamics of intertidal megaripples in the Mawddach Estuary, North Wales, and the implications for palaeoflow reconstructions
Authors: LARCOMBE PIERS , JAGO COLIN F. 
Published: November 2003

ABSTRACTThe morphology and migration rate of tidal bedforms are important because of their use in interpretation of modern and ancient sediment transport regimes. Tidal flow, megaripple morphology and migration were studied in the mesotidal Mawddach Estuary, North Wales, to examine the veracity of published flow‐bedform relationships, quantify spatial variations in migration and assess consequences for palaeoflow reconstruction. Two transects were surveyed along a megarippled intertidal shoal (mean grain size 280 μm) for a period of 22 semi‐diurnal tidal cycles. A vertical array of current meters recorded tidal current profiles at the centre of one of the transects.Flood tidal currents dominate at Fegla Fach shoal, with peak velocities over 1 m s−1 at spring tides, and 0.5 m s −1 at neaps, and bed sediment transport was also flood‐dominated. Over the lunar cycle, the morphology of the megaripples on the survey lines was divisible into three phases:1 the neap mode‐consisting of near‐moribund two‐dimensional (2‐D) flood‐orientated megaripples of wavelength c. 6 m and height c. 0.2 m;2 a transitional mode‐where, on rising tidal ranges, scour pits formed and developed into 3‐D megaripples which underwent net migration with the flood tide;3 the spring mode‐consisting of 3‐D megaripples of wavelength c. 4 m and height c. 0.2 m. Despite complete re‐orientation by the ebb tide, these were recognizable from one low water survey to the next, and net migration was c. 1 m per tide with the flood tide. We infer the presence of the equilibrium ‘spring tidal form’ occurring as flood‐orientated megaripples during the flood tide.The data support previously reported separation of 2‐D and 3‐D megaripples at a depth to grain size ratio of 8000, and at a depth‐mean velocity of the dominant tide (Umaxdom) of 0.75‐0.8 m s−1. A migration threshold exists at Umaxdom of c. 0.53‐0.57 m s−1.Measures of migration which might be used on preserved sections have been applied to the data. These measures systematically overestimated bedform migration at most stages of the lunar cycle (by <25% at spring tides and <140% in the post‐spring transition period), but were accurate when the megaripples had developed into their 'spring tidal form’. There is significant variation of migration rates within the survey populations. We conclude that whilst the occurrence of megaripple cross‐sets may be used as a palaeoflow indicator, and sedimentary structures associated with 2‐D to 3‐D transitions may also be indicative of palaeoflows, there are likely to be significant uncertainties involved in using tidal bundles as an indication of sediment transport rates.
Geostrophic sand ridge, dune fields and associated bedforms from the Northern KwaZulu‐Natal shelf, south‐east Africa
Authors: RAMSAY PETER J. , SMITH ALAN M. , MASON THOMAS R. 
Published: November 2003

ABSTRACTSubaqueous dunes are formed on the KwaZulu‐Natal outer‐shelf due to sediment transport by the Agulhas Current (geostrophic current). These dunes occur within two dune fields at depths of − 35 to − 70 m. The net sediment transport direction is south, but short‐period reversals form northward‐migrating bedforms. The dune fields are physically bounded by late Pleistocene beachrock and aeolianite ledges. A bedform hierarchy has been recognized in the dune fields comprising a system of three generations of climbing bedforms. The outer dunefield has given rise to a sand ridge (H=12 m; L=4 km; W=1.1 km; and an 8° lee slope) whereas the inner dune fields have achieved large‐scale dune status. Bedload parting zones within the dune fields occur where the sediment transport direction switches from north to south due to reversals in the geostrophic flow; these zones occur at depths of − 60, − 47 and − 45 m. An interpretative stratigraphic model is presented on what such geostrophite deposits would look like in the ancient sedimentary record.
Frequency spectra and intermittency of the turbulent suspension process in a sand‐bed river
Authors: LAPOINTE MICHEL F. 
Published: November 2003

ABSTRACTPrevious research suggests that the turbulence‐driven suspension process in sand‐bed channels is dominated by intermittent, energetic eddies with length scales of the order of channel depth. Because of the scarcity of data on the turbulent suspension process in alluvial channels, the possible variability in suspension intermittency and turbulent frequency content due to contrasts in flow depth, velocity or bedforms remains unclear however. The present study analyses eddy correlation suspension signals from seven deployments in varied flow conditions around a sandy meander bend. Deployment depths at near‐bankfull flow stages varied from 2 to 5.5 m, velocities at 0.75–1 m height from 0.6 to 0.9 m s−1 and local mean suspended sand concentrations ranged from 30 to 150 mg L−1 in the intermittence and spectral content of sand suspension between the various deployments are analysed and results are compared with previously published findings. Study data suggest that the dominant eddy sizes involved in sediment mixing across the sensor level are consistently of the order of 1–5 times flow depth and lie within the ‘energy‐bearing’ turbulent range. When sand suspension is analysed in the time domain in the various deployments, energetic, burst‐like suspension events occupying only 1‐5% of the record duration account for 20‐90% of the suspension work. The degree of intermittence in the suspension process was observed to increase in deeper flows, where mixing events contributing extreme vertical sediment fluxes appear to be relatively more frequent.
An interplay of syn‐ and intereruption depositional processes: the lower part of the Jangki Group (Miocene), SE Korea
Authors: BAHK J. J. , CHOUGH S. K. 
Published: November 2003

ABSTRACTThe lower part of the Jangki Group (Miocene), SE Korea consists of pyroclastic mass‐flow‐dominated facies and epiclastic stream‐flow‐dominated facies which reflect sedimentation during syn‐ and intereruption periods, respectively. On the basis of pyroclastic composition, sedimentary structures and bed geometry, they are organized into two facies associations: (1) dacitic and basaltic debris‐flow and hyperconcentrated‐flood‐flow deposits of eruption periods, and (2) epiclastic stream‐flow and interchannel deposits of intereruption periods. The lateral relationship between the syn‐ and intereruption deposits varies significantly over short distances (2 km). In the western part of the study area, syneruption deposits are predominant, and fluvial deposits occur as small‐scale channel‐fill gravelstone bodies encased within dacitic debris flow deposits. In the eastern part, however, intereruption deposits are dominated with thick sequences of interbedded channel and interchannel deposits. The abrupt lateral change indicates alternation of epiclastic axial fluvial system with pyroclastic‐rich volcaniclastic aprons. The syneruption deposits are enriched in vitric ash but lack contemporary volcanic rock fragments (dacitic or basaltic). They are sharply differentiated from intereruption deposits that mostly consist of epiclasts and are deficient in vitric ash. The vertical transition suggests that streams drained a hinterland of igneous basement rocks during intereruption periods and became bulked with pyroclasts during syneruption periods.
Fibrous calcite from the Ordovician of Tennessee: preservation of marine oxygen isotopic composition and its implications
Authors: TOBIN KENNETH J. , WALKER KENNETH R. , STEINHAUFF D. MARK , MORA CLAUDIA I. 
Published: November 2003

ABSTRACTThree categories of fibrous calcite from early to middle Caradoc platform‐marginal buildups in east Tennessee can be delineated using cathodoluminescent microscopy, minor element chemistry and stable C‐O isotopic composition. Bright luminescent fibrous cement has elevated Mn (>1000 p.p.m.), negative δ13C and intermediate δ18O values relative to other types of fibrous calcite. This cement reflects fibrous calcite that interacted with reducing Mn‐rich fluids. Dully luminescent fibrous cement has elevated Fe (>400 p.p.m.), positive δ13C and negative δ18O values relative to other fibrous cements. This cement was stabilized by burial fluids. Nonluminescent fibrous cement has low Mn and Fe (generally below 400 p.p.m.) and positive δ13C and δ18O values relative to other types of fibrous calcite. The latter cement is interpreted to be the best material for determining the isotopic composition of calcite precipitated in equilibrium with early to middle Caradoc seawater, which is δ13C=1% PDB and δ18O=−4 to −5‰ PDB. Results from this study and Ashgillian brachiopods indicate that the average δ18O composition of the Ordovician ocean, during nonglacial periods, was probably never more negative than −3‰ SMOW. Assuming an Ordovician seawater δ18O value of −1‰ SMOW, Holston Formation fibrous cements would have precipitated at temperatures between 27 and 36 °C, which is near the upper temperature limit for metazoans. A seawater δ18O value of −2‰ SMOW yields temperatures ranging from 23 to 31 °C, while a −3‰ SMOW value yields temperatures of 18–26 °C.
Experimental turbidity currents entering density‐stratified water: analogues for turbidites in Mediterranean hypersaline basins
Authors: RIMOLDI BIANCA , ALEXANDER JAN , MORRIS STEPHEN 
Published: November 2003

ABSTRACTExperimental turbidity currents entering two‐layer density‐stratified water behave differently from similar currents flowing over the same topography into non‐stratified water. Experiments were designed as analogues for flows entering Mediterranean hypersaline pools. In both the hypersaline pools and the experiments, the water density changes abruptly across a pycnocline. Turbidity currents generated on a platform at the level of the pycnocline behaved in one of three ways as they flowed from the platform into deeper stratified water. (1) When the bulk density of the current was less than the dense water layer, the current spread at the pycnocline. The head of the current advanced rapidly when it lost contact with the bed. Grains settling out of the current fell through the dense water layer forming an extensive deposit. In nature this behaviour will lead to ‘turbidites’ with sharp but non‐erosive bases, strongly developed grading and no traction features. (2) When the bulk density of the current was greater than the dense water layer, the current continued as an underflow, plunging into the deeper water. Sedimentation lowered the bulk density of the current and the low‐density interstitial fluid caused the head to loft. Low‐density interstitial fluid convected from the body of the current, lofting particles into the water column. These particles were hydraulically sorted during upward transport and subsequent settling to the floor. The resulting turbidites had a more limited extent than the deposits of either non‐lofting underflows or interflows. By inference from the experiments, natural deposits of this type may have local (proximal) erosion and traction features at the base and strongly graded tops. (3) In some of the currents with high bulk density, the rising turbid water reached the pycnocline and spread at that level as a secondary interflow. The tail of the turbidity current, which was less dense than the head and body of the current, flowed above the pycnocline adding momentum to the secondary interflow. The thin non‐erosive graded deposit from the secondary interflow may extend beyond the deposits of the primary underflow. In all three cases (but more pronounced in cases 2 and 3) the interaction of the current with the pycnocline displaced that surface and generated a wave that was reflected back and forth from each end of the pool. The waves remobilized sediment on the ramp.
Sedimentology of Badenian (middle Miocene) gypsum in eastern Galicia, Podolia and Bukovina (West Ukraine)
Authors: PERYT TADEUSZ MAREK 
Published: November 2003

ABSTRACTPrimary gypsum is the main evaporite mineral in the middle Miocene (Badenian) of the West Ukraine. The lower part of the gypsum sequence is built of autochthonous gypsum while the upper part is composed of allochthonous gypsum that formed following a major, tectonically induced, change in basin morphology. This change resulted in the destruction of the gypsum deposited on the margins of the basin and formation of redeposition features.Autochthonous gypsum facies were deposited in two main environments: (1) giant gypsum intergrowths precipitated from highly concentrated brines; (2) very shallow subaqueous gypsum deposited in a vast brine pan. The brine pan was characterized by a facies mosaic that reflects an interplay of concentrated brines from the central part of the evaporite basin and diluted brines due to the influx of continental meteoric waters. The facies continuum, microbial gypsum ‐ bedded selenite ‐ massive selenite ‐ sabre gypsum, indicates increasing salinity of the brine with time. This type of facies pattern has been established in recent salinas that are analogous to Badenian gypsum in their lateral facies changes. However, the pattern of facies distribution with respect to the open sea in the Badenian basin is opposite to that found in recent salinas.The pattern of the Badenian gypsum facies in the Ukraine indicates that facies repetition may have been related to climatically controlled salinity changes and not to depth changes, as is commonly used to explain the repetition of sulphate facies in a vertical succession.
Grain‐size characteristics of fine‐grained unflocculated sediments I:‘one‐round’ distributions
Authors: KRANCK KATE , SMITH P. C. , MILLIGAN T. G. 
Published: November 2003

ABSTRACTA simple physical model of gravitational settling from an unsorted, unflocculated source suspension is presented and an equation derived to describe the grain‐size spectra of the resulting bottom sediment. Results of grain‐size analyses of sediments from a variety of environments and geographical locations are shown to conform with the postulated model. The characteristic size spectrum, termed ‘one‐round’ sediment, identifies a deposit which has settled from suspension with no subsequent reworking resulting in modification of the grain‐size distribution. The distribution of settling rates of grains in the suspension may be inferred from an analytical form fit to the bottom sediment grain‐size spectrum, along with knowledge of certain physical characteristics of the fluid (e.g. mean velocity profile).
Significance of fan deltas without toe‐sets within rift and piggy‐back basins: examples from the Corinth graben and the Mesohellenic trough, Central Greece
Authors: ZELILIDIS ABRAHAM , KONTOPOULOS NIKOLAOS 
Published: November 2003

ABSTRACTTrapezoidal‐type fan deltas, lacking bottomset deposits, were studied in two different tectonic settings: extensional (rifted), and compressional (piggy‐back) basins. In both cases studied fan deltas were characterized by: (i) an absence of bottomsets; (ii) development in protected or narrow basins and sub‐basins confined by intrabasinal basement highs or by topographic highs, respectively; (iii) coarse‐grained sediment fluxes, dominated by mass‐flows forming fan deltas that prograded from steep nearshore slopes basinwards; and (iv) a high‐energy environment, with powerful underflows that probably bypassed the basins and transported fine‐grained sediments outside the basins. The location of channels cut by such underflows is influenced by local tectonic style. When the supplied sediments in the rifted basins overstepped the intrabasinal basement highs, trapezoidal fan deltas were replaced upwards by Gilbert‐type deltas, with bottomsets.
Facies and cyclicity of the Late Permian Bainmedart Coal Measures in the Northern Prince Charles Mountains, MacRobertson Land, Antarctica
Authors: FIELDING CHRISTOPHER R. , WEBB JOHN A. 
Published: November 2003

ABSTRACTThe Late Permian Bainmedart Coal Measures form part of the Permo‐Triassic Amery Group, which crops out in the Beaver Lake area of the Northern Prince Charles Mountains, MacRobertson Land, Antarctica. The exposed strata are believed to have formed in graben or half‐graben sub‐basins on the western edge of the Lambert Graben, a major failed rift system. Sedimentological analysis has revealed that these rocks formed in alluvial environments in which swiftly flowing rivers of low sinuosity (represented by Facies A1 and A2) flowed northward down the axis of the basin, and were associated with waterlogged floodbasin and peat‐forming wetlands (Facies B1‐B4). A third Facies Association (comprising Facies C1‐C3), interpreted as the deposits of lake floor and delta environments, is exclusively developed within a distinctive, fine‐grained interval here named the Dragon's Teeth Member. The proportion of Association B facies within the succession increases markedly above the level of the Dragon's Teeth Member (at about 300 m above the base of the formation).Flat, low‐angle and undulatory bedding structures preserved within channel deposits are suggestive of sediment deposition in flow conditions which were often critical or supercritical. Presence of massive and chaotic intervals of sandstone further implies some deposition from high‐concentration aqueous flows. Alluvial channel bodies show evidence of incision into underlying substrates, both during initiation and at later stages in channel belt construction. The lack of interfingering between channel deposits and coals suggests that thick peats formed only in areas and at times of minimal clastic sediment supply.Analysis of well‐developed cyclicity within the coal measures suggests that the dominant control on sequence architecture was climatic, related to precessional Milankovitch fluctuations of c. 19‐kyr periodicity. Cycles began abruptly with the deposition of coarse‐grained material in high‐energy alluvial channels, which contracted with time in response to changes in water supply (rainfall). Upper parts of cycles are dominated by finer‐grained sediments and then coal, indicative of progressively reduced coarse sediment input. Tectonic processes overprinted this pattern at least once during the period of sediment accumulation, to form the Dragon's Teeth Member.
Morphology and growth of aragonite crystals in hot‐spring travertines at Lake Bogoria, Kenya Rift Valley
Authors: JONES BRIAN , RENAUT ROBIN W. 
Published: November 2003

ABSTRACTPseudohexagonal aragonite crystals are common components in some hot‐spring travertines at Chemurkeu on the western shore of Lake Bogoria, Kenya. Beds, lenses and pods of aragonite crystals are intercalated with beds of white non‐crystallographic calcite dendrites. The pseudohexagonal aragonite crystals, which are up to 4 cm long and 4 mm wide, are formed of nested skeletal crystals. Each skeletal crystal is formed of cyclical twinned crystals that are constructed of stacked subcrystals. The latter are inclined at a consistent angle of 40° to the long axis of the pseudohexagonal aragonite crystal. Intense competition for space during growth modified the crystal morphology with the result that many of the pseudohexagonal crystals are distorted. Intercrystalline and intracrystalline pores are filled or partly filled by epitaxial aragonite overgrowths and/or reticulate microbial coatings that have a high concentration of Si and Mg. In places, this extracellular mucus induced etching of the underlying aragonite crystal.Today the hot (T>95 °C) Na‐HCO3‐Cl spring waters at Chemurkeu have a salinity of 5–6 g L−1 TDS, a pH of 8·1–9·1, Ca2+ concentrations of <2 mg L−1 and Mg2+ concentrations of <0·7 mg L−1, The springs of the Lake Bogoria Geothermal Field are fed by a shallow aquifer (T∼100 °C) and a deeper aquifer (T∼170 °C). Springs at Chemurkeu derive from meteoric groundwater, lake water and condensed steam, and are fed mainly from the shallow thermal aquifer. Much of the aragonite may have formed when the spring waters contained more dissolved Ca2+ than today, possibly under more humid conditions during the Holocene.
Holocene Saharan dust deposition on the Cape Verde Islands: sedimentological and Nd‐Sr isotopic evidence
Authors: ROGNON P. , COUDÉ‐GAUSSEN G. , REVEL M. , GROUSSET F. E. , PEDEMAY P. 
Published: November 2003

ABSTRACTHolocene aeolian silts deposited on the Cape Verde Islands provide information about the origin of African palaeodusts that have fallen on the north‐eastern Atlantic ocean over the last 10 000 years. Sedimentological composition indicates that most of these aeolian silts are unquestionably of continental origin. Their Sr and Nd isotopic composition identifies a Saharan origin‐suggesting transport by Harmattan winds. We estimate that Saharan dust comprises 75–95% of material in these Holocene silts, the rest coming from the weathering of local basaltic bedrock.
Sedimentological responses to basin initiation in the Devonian of East Greenland
Authors: MARSHALL J. E. A. , STEPHENSON B. J. 
Published: November 2003

ABSTRACTThe Devonian of East Greenland comprises a thick sequence of continental clastic sediments infilling an extensional basin. West of the main basin bounding fault (Western Fault Zone) are scattered outliers of Devonian conglomerate which accumulated in small basins such as found on the island, Ella Ø. The Ella Ø Basin formed by extensional movement along the Narhval Sund Fault accompanied by the formation of a prominent bedding parallel detachment surface which was subsequently modified by sub‐aerial exposure to become the unconformity surface. Mapping of this unconformity surface shows major vertical relief. A thick sequence of conglomerate occurs on Ella Ø, which, close to its exposed basal unconformity, has three lacustrine beds intercalated within it. Detailed analysis of one lake unit shows it to have significant lateral variation. At proximal localities it largely comprises lacustrine turbidites, whereas more distal locations were within a stratified lake. The interpreted sequence of events on Ella Ø is an interval of fluvial sedimentation followed by rapid drowning of the topography with lacustrine sediments onlapping onto basement. After an interval of deeper lacustrine sedimentation including laminites, the lake shallowed, the lithology changed to limestone and the lake dried out. Conglomerate deposition then recommenced. Maximum lake water depth of 100 m is estimated following correction (7°) for post‐Devonian rotation, both determined using the distribution of lacustrine sediments on the unconformity surface. The preferred mechanism for flooding of the Ella Ø Basin is episodic flooding of the entire basin system. Lacustrine sediment preservation results purely from its deposition amongst topography at the edge of the active fluvial system. In such fluvial systems lacustrine sediments may be part of the normal sedimentary cycle but were almost always removed by reworking.