bluuebunny/crossref_metadata_2025_split huggingface dataset:
Sedimentation in a river dominated estuary
Authors: COOPER J. ANDREW G. 
Published: June 2006

ABSTRACTThe Mgeni Estuary on the wave dominated east coast of South Africa occupies a narrow, bedrock confined, alluvial valley and is partially blocked at the coast by an elongate sandy barrier. Fluvial sediment extends to the barrier and marine deposition is restricted to a small flood tidal delta.Sequential aerial photography, sediment sampling and topographical surveys reveal a cyclical pattern of sedimentation that is mediated by severe fluvial floods which exceed normal energy thresholds. During severe floods (up to 10x 103 m3 s−1), lateral channel confinement promotes vertical erosion ofbed material. Eroded material is deposited as an ephemeral delta in the sea. After floods the river gradient is restored within a few months through rapid fluvial deposition and formation of a shallow, braided channel. Over an extended period (approximately 70 years) the estuary banks and bars are stabilised by vegetation and mud deposition. Subsequent downcutting in marginal areas transforms the channel to an anastomosing pattern which represents a stable morphology which adjusts to the normal range of hydrodynamic conditions. This cyclical pattern of deposition produces multiple fill sequences in such estuaries under conditions of stable sea level.The barrier and adjacent coastline prograde temporarily after major floods as the eroded barrier is reformed by wave action, but excess sediment is ultimately eroded as waves adjust the barrier to an equilibrium plan form morphology. Deltaic progradation is prevented by a steep nearshore slope, and rapid sediment dispersal by wave action and shelf currents.During transgression, estuarine sedimentation patterns are controlled by the balance between sedimentation rates and receiving basin volume. If fluvial sedimentation keeps pace with the volume increase of a basin an estuary may remain shallow and river dominated throughout its evolution and excess fluvial sediments pass through the estuary into the sea. Only if the rate of volume increase of the drowned river valley exceeds the volume of sediment supply are deep water environments formed. Under such conditions an estuary becomes a sediment sink and infills by deltaic progradation and lateral accretion as predicted by evolutionary models for microtidal estuaries. Bedrock valley geometry may exert an important control on this rate of volume increase independently of variations in the rate of relative sea level change.If estuarine morphology is viewed as a function of the balance of wave, tidal and fluvial processes, the Mgeni Estuary may be defined as a river dominated estuary in which deltaic progradation at the coast is limited by high wave energy. It is broadly representative of other river dominated estuaries along the Natal coast and a conceptual regional depositional model is proposed. Refinement of a globally applicable model will require further comparative studies of river dominated estuaries in this and other settings, but it is proposed that river dominated estuaries represent a distinct type of estuarine morphology.
Formation of braided river floodplains, Waimakariri River, New Zealand
Authors: REINFELDS IVARS , NANSON GERALD 
Published: June 2006

ABSTRACTFloodplains along the braided gravel‐bed Waimakariri River are discontinuous although generally extensive landforms composed predominantly of gravel bars capped with vertically accreted fines. In order of importance, three mechanisms lead to floodplain formation. River bed abandonment by lateral migration of the braid‐train initiates the formation of the largest floodpiains, which usually occur downstream of tributary fans and valley bedrock spurs. In the headwater reaches, localized river bed aggradation during high magnitude events leads to floodplain formation by producing a surface resistant to erosion by lesser events due to either the coarse nature of the deposit, or by its elevation over the river bed, or both. The least important mechanism initiating floodplain development is localized river or channel incision. A six stage model is proposed for the sequential development of floodplains on the Waimakariri River from: (i) active river bed; (ii) stabilizing bar; (iii) incipient floodplain; (iv) established floodplain; (v) mature floodplain; and to (vi) terrace. Two mechanisms, lateral migration by the braid‐train and reactivation of abandoned channels within floodplains, operating separately or in combination, are responsible for floodplain reworking and their relatively young age (<250 years). Clearly, braided rivers can construct substantial areas of well developed floodplain.
Drag reduction in turbulent muddy seawater flows and some sedimentary consequences
Authors: BEST J. L. , LEEDER M. R. 
Published: June 2006

ABSTRACTTurbulent saline flows which contain low concentrations of suspended clay (< 10 g 1−1) have previously been reported to possess dramatically different boundary layer characteristics from clearwater flows and to exhibit the phenomenon of drag reduction. Drag reduction causes significantly lower friction factors and higher erosion thresholds in muddy saline flows than predicted by experimental data gained from fresh‐water flows using the law of the wall to estimate bed shear stress. Confirmation is given by experimental results on drag reduction using non‐intrusive laser Doppler measurements in seawater flows with <2·2 g 1−1 maximum concentration of suspended clay. Increasing the concentration of clay caused progressively lower velocities near the wall due to a gradual thickening of the buffer region of the turbulent boundary layer. Several aspects of sediment transport, deposition and bedform development in natural marine environments are thus considered to be significantly affected by the drag reduction process.
Physico‐chemical environment of pedogenic carbonate formation in Devonian vertic palaeosols, central Appalachians, USA
Authors: DRIESE STEVEN G. , MORA CLAUDIA I. 
Published: June 2006

ABSTRACTThe morphology and geochemistry of pedogenic carbonate found in vertic claystone palaeosols in the Devonian Catskill Formation in central Pennsylvania preserve a record of the physical and chemical environment of carbonate precipitation. The carbonate is characterized by three distinct petrographic generations. Pedogenic rhizoliths and nodules are the earliest precipitated generation, and typically consist of dull red‐brown luminescent micrite. Clear, equant calcite spar cement fills voids in the centres of rhizoliths, as well as circumgranular cracks and septarian voids in nodules. Early spar cements are non‐luminescent to dull luminescent, whereas later spar cements exhibit bright yellow‐orange luminescence. Late stage pedogenic fractures are always occluded with very bright yellow‐orange luminescent spar cements. The incorporation of progressively higher concentrations of Mn (up to 34000 ppm) into successively younger calcite spar cements, without concomitant increases in Fe, suggests carbonate precipitation from an evolving meteoric water in which Mn2+ became increasingly mobile over time. The increased mobility is possibly due to decreasing Eh, resulting from oxidation of organic matter after rapid soil burial on the floodplain. The amount of Fe2+ available for incorporation into calcite was limited because most iron was immobile, having been earlier oxidized and bound to the palaeosol clay matrix as a poorly crystallized ferric oxide or oxyhydroxide mineral. Carbon isotope compositions of pedogenic carbonate correlate with the inferred depth of carbonate precipitation. Rhizoliths preserved below the lowest stratigraphic occurrences of pedogenic slickensides are consistently depleted in 13C relative to nodules, which formed stratigraphically higher, within the zone of active soil shrink and swell processes. Nodular carbonate, precipitated in proximity to deep cracks in the soil, is enriched due to increased gas exchange with isotopically heavy atmospheric CO2. Accordingly, rhizolith compositions will most accurately estimate palaeoatmospheric levels of CO2; the use of nodule compositions may result in overestimation of PCO2 by as much as 30%.
Combined flow origin of edgewise intraclast conglomerates: Sellick Hill Formation (Lower Cambrian), South Australia
Authors: MOUNT JEFFREY F. , KIDDER DAVID 
Published: June 2006

ABSTRACTUnusually thick, coarse grained edgewise intraclast conglomerates occur at eight or more horizons within subtidal nodular and ribbon bedded wackestones and packstones of the Lower Cambrian Sellick Hill Formation, South Australia. The intraclast beds are flat based and laterally discontinuous, forming bar‐like structures that must have exhibited bathymetric relief of as much as 1 m. The internal fabrics of these beds are variable. Thinner beds are dominated by flat‐lying intraclasts; thicker beds contain both chaotic, randomly oriented, steeply inclined intraclasts and clusters of fan‐shaped, vertically stacked edgewise intraclasts.The Sellick Hill Formation intraclast conglomerates are inferred to have been formed by intense, storm‐generated combined flows on a broad, subtidal carbonate ramp. Superimposition of wave‐induced oscillatory motions on geostrophic bottom flows during large storms generates short‐lived, but exceptionally high instantaneous shear stresses in the bottom boundary layer. Entrainment of the relatively large intraclasts occurs through sliding, rather than pivoting. Edgewise fabrics are a product of asymmetric acceleration and deceleration of intraclasts during passage of waves and the chaotic nature of collisions between intraclasts moving within the boundary layer. Collisions between intraclasts impart a rotating moment, causing intraclasts to tip up during maximum fluid shear stress. Lodgement or packing of clasts in vertical or steeply inclined positions occurs within scours, where intraclasts can wedge between other vertically inclined clasts, or where intraclasts are pinned in steep orientations by collisions with shallowly inclined intraclasts. Differential erosional resistance of the intraclast deposits probably led to the development of sharp lateral changes in thickness.The Sellick Hill Formation intraclast conglomerates record erosion and reworking of subtidal, subfairweather wave base environments by exceptionally intense and presumably rare storm flows. The intraclast horizons represent a substantial loss in stratigraphic resolution due to widespread erosion of the ramp.
Depositional framework and controls on mixed carbonate‐siliciclastic gravity flows: Pennsylvanian‐Permian shelf to basin transect, south‐western Great Basin, USA
Authors: MILLER ROBERT P. , HELLER PAUL L. 
Published: June 2006

ABSTRACTMixed carbonate‐siliciclastic sediment gravity flow deposits of Late Pennsylvanian to Early Permian age are exposed in the Death Valley ‐ Owens Valley region of east‐central California. The Mexican Spring unit constitutes the upper part of the Keeler Canyon Formation and is characterized by turbidites, debris flow deposits and megabreccias, all of mixed carbonate‐siliciclastic composition. The mixed composition of the Keeler Canyon Formation provides an opportunity to link facies architecture to controls on depositional system development. Depositional relationships indicate that the deposits represent a non‐channellized base of slope carbonate apron system with inner, outer and basinal facies associations. These gravity flow deposits are characterized by repeated stacked, small scale (<15 m) coarsening and thickening upward cycles with superimposed medium scale (>100 m) coarsening and thickening upward cycles.Contemporaneous outer shelf and upper slope deposits of the Tippipah Limestone are exposed at Syncline Ridge on the Nevada Test Site. The deposits consist of carbonate buildups directly overlain by cross bedded, quartz‐rich sandstone and conglomerate which filled channels that traversed across the previously existing carbonate shelf. Detritus was transported to the west, down the upper slope by gully systems that fed the temporally persistent base of slope apron of the upper part of the Keeler Canyon Formation. This style of deposition differs from point‐sourced siliciclastic submarine fan depositional systems. However, the Keeler Canyon system has lithofacies similar to some sandy siliciclastic turbidite systems, such as the delta‐fed submarine ramp facies model, which is a line‐sourced, shelf‐fed system that is not supply limited.The mixed clastic apron systems of the Keeler Canyon Formation differ from classical carbonate aprons in that the former is characterized by an abundance of sedimentary cycles. Controls on the development of these cycles and of the facies distribution may have resulted from changes in type and rate of sediment supply, relative sea level changes and/or tectonic events. Interpretation of the data is focused on relative changes in sea level as the most significant control on development of the depositional system. Relative sea level changes serve two important functions: (1) they provide a mechanism for bringing coarse siliciclastic and bioclastic grains together on the outer shelf, and (2) shelf margin collapse may be initiated during relative lowstands allowing for transport of the sediment to the deep basin and development of deep basinal cycles. Therefore, an abundance of mixed clastic gravity flow deposits such as these in the rock record may be an indicator of periods of high frequency changes in relative sea level, which is a characteristic of Late Palaeozoic sea level history.
Biofilms, microbial mats and microbe‐particle interactions: electron microscope observations from diatomaceous sediments
Authors: WESTALL F. , RINCÉ Y. 
Published: June 2006

ABSTRACTSediments and diatoms from the mudflats of the Bay of Bourgneuf in western France were examined in an electron microscope study of biofilms and microbial mats. The sediments were kept in an aquarium for study and a diatom culture was made of the benthic diatoms. The sediment biofilm was composed of exopolymeric substances (EPS), incorporated clay particles and, rarely, bacteria. This film coated all particles at the sediment‐water interface. Its surface morphology reflected its composition and internal structure. Thin films were smooth, whilst a lumpy structure or incorporated fibrils produced either a mammillated or ropy surface, and clays in the structure gave rise to a flaky morphology. At shallow depths in the sediment column (0.5 cm) the biofilm was already degraded. The biofilm coating degraded diatom frustules in the benthic diatom culture consisted of EPS and bacteria and presented a ragged appearance.Microbial mats occurred on the surface of the fresh littoral sediments as well as those in the aquarium, and on the wall of the aquarium. The mat on the surface of the aquarium sediments had an open structure with webs of fibrils and bacteria in the pore space. It formed in a relatively quiet environment. Pore space was more limited in the mat from the surface of the fresh littoral sediments, in which direct contact between biofilm coated particles was common. In the exposed environment of the aquarium wall there was a thick, resistant coating of EPS.In addition to binding particles together, the presence of mats and biofilms in sediments affects sediment physical properties such as porosity and permeability, the flux of dissolved substances in pore waters and the dissolution of particles and can, therefore, influence early diagenesis. Mats and biofilms seem to be more readily preserved in the geological record than the micro‐organisms, such as bacteria, which produce them. Their identification in the sedimentary record would greatly aid interpretation of sediment genesis and evaluation of the microbial role in sediment formation.
A flume study on the development and equilibrium morphology of current ripples in very fine sand
Authors: BAAS JACO H. 
Published: June 2006

ABSTRACTAn empirical model is constructed for the development and equilibrium dimensions of small scale, unidirectional bedforms in sand with a median grain size of 0·095 mm, based on a series of steady flow experiments in a flume. Current ripples always attain a linguoid plan morphology with constant average height (13·1 mm) and wavelength (115·7 mm), provided that sufficient time is allowed for their formation. The development pattern of these ripples on a flat bed is independent of flow velocity, and involves four stages: (1) incipient ripples; (2) straight and sinuous ripples; (3) non‐equilibrium linguoid ripples, and (4) equilibrium linguoid ripples. Straight and sinuous ripples are non‐equilibrium bedforms at all flow velocities. The time needed to reach equilibrium dimensions is related to the inverse power of flow velocity and ranges from several minutes to more than hundreds of hours. At flow velocities where washed ripples are stable, the equilibrium wavelength is similar to that of equilibrium linguoid ripples, but the equilibrium height rapidly decreases from 13·1 mm to zero towards upper stage plane bed conditions.The results of the flume experiments correspond reasonably well with those of previous studies, provided that various complicating factors, such as different experimental methods, different sediment characteristics, shallow flow depths and non‐equilibrium runs, are accounted for.
Tuffaceous ephemeral lake deposits on an alluvial plain, middle Tertiary of central California
Authors: BARTOW J. ALAN 
Published: June 2006

ABSTRACTThe Oligocene and Miocene Valley Springs Formation is one element in the middle to late Cenozoic alluvial fill of the Central Valley of California and represents a large fluvial depositional system that extended westward from sediment‐filled palaeovalleys in the high Sierra Nevada to a piedmont alluvial plain under the present Central Valley. The Valley Springs Formation consists largely of tuffaceous mudrock, tuffaceous sandstone, polymict conglomerate and rhyodacitic tuff. The most prominent lithofacies in the formation is yellowish grey or greyish yellow, tuffaceous mudstone and claystone characterized by crude, commonly wavy layering or bedding, an irregular fracture, and common clay‐lined partings, fissures, and small branching tubules.Petrographic and X‐ray diffraction analyses of the composition, texture, fabric, mineralogy and organic content of an 8 m thick section of the tuffaceous mudrock lithofacies have provided evidence for an origin quite different from the earlier interpretations of this lithofacies as altered tuffs. Numerous rounded mudstone or claystone clasts in a detrital mud matrix, together with abundant freshwater microfossils (megaspores, chrysophyte cysts, diatoms and sponge spicules), indicates that the lithofacies is a mostly epiclastic sedimentary deposit. Root traces, fissuring, orientated clay skins lining pores and secondary surfaces, and mineralogical data are evidence of periodic exposure and desiccation.The tuffaceous mudrock lithofacies of the Valley Springs Formation, interpreted in the context of the middle to lower regions of a piedmont alluvial system, most probably represents a complex of ephemeral lake and marsh environments on a low gradient alluvial plain. The inferred abundance of shallow lakes, ponds and marshes implies a climate that was wetter than the semi‐arid climate of the region today.
High energy coralgal‐stromatolite frameworks from Holocene reefs (Tahiti, French Polynesia)
Authors: CAMOIN GILBERT F. , MONTAGGIONI LUCIEN F. 
Published: June 2006

ABSTRACTDrill cores from Holocene reefs on Tahiti (French Polynesia) reveal a framework composed of massive branching acroporids encrusted by coralline algae associated with sessile vermetid gastropods and arborescent foraminifers. Laminated micritic crusts form coatings over coral branches or, more commonly, over related encrusting organisms throughout the cored reef sections; these crusts appear as a major structural and volumetric component of the reef framework. The microbial nature of these micritic crusts is inferred from their typical organic growth forms and geometry, the occurrence of microbial remains and stable isotope measurements. The reef communities accumulated at depths less than 5 m below mean sea level in a high energy environment throughout vertical growth from 7140 ± 170 yr bp to the present. The nature of the involved benthic communities, stable isotope data and high calcification rates of microbially encrusted corals strongly suggest that local environmental conditions have been optimal for reef development for the last 7000 years. The causes of the predominance of microbial communities over actual encrusters (red algae, foraminifers) remain problematic and could be related to short term fluctuations in ecological parameters. Microbial micritic crusts seemingly played a prominent role in protecting the coralgal colonies from bioeroders and grazers and, possibly, in strengthening the framework, due to rapid lithification. The record of similar microbial crusts in other Quaternary reef tracts suggests that microbial communities may have played a more prominent role in Quaternary reefs than presently recognized.
A note on the use of statistics in reporting detrital clastic compositions
Authors: HOWARD JEFFREY L. 
Published: June 2006

ABSTRACTError polygons and error bars are commonly used graphical methods for showing the precision associated with means on triangular diagrams and orthogonal plots, respectively. If the error or precision associated with a mean, based either upon counts of individual components or upon count ratios, is to be defined using basic statistics, the confidence interval method should be used. Standard deviation alone, which has been frequently in the literature, is not an appropriate statistic to use for this purpose. The graphical use of confidence intervals permits a rapid visualization of statistically significant differences between means, and thus more quantitative description of detrital clastic composition. Previous investigators using the standard deviation method have underestimated the precision of their data when sample sizes are large, and overestimated precision with small samples.