			Home About us Contact

Grain Size Distribution (grain + size_distribution)

Distribution by Scientific Domains

Earth and Environmental Science	52%
Engineering	14%
Chemistry	9%

Selected Abstracts

Sedimentological, modal analysis and geochemical studies of desert and coastal dunes, Altar Desert, NW Mexico

EARTH SURFACE PROCESSES AND LANDFORMS, Issue 4 2007
J. J. Kasper-Zubillaga
Abstract Sedimentological, compositional and geochemical determinations were carried out on 54 desert and coastal dune sand samples to study the provenance of desert and coastal dunes of the Altar Desert, Sonora, Mexico. Grain size distributions of the desert dune sands are influenced by the Colorado River Delta sediment supply and wind selectiveness. The desert dune sands are derived mainly from the quartz-rich Colorado River Delta sediments and sedimentary lithics. The dune height does not exert a control over the grain size distributions of the desert dune sands. The quartz enrichment of the desert dune sands may be due to wind sorting, which concentrates more quartz grains, and to the aeolian activity, which has depleted the feldspar grains through subaerial collisions. The desert dune sands suffer from little chemical weathering and they are chemically homogeneous, with chemical alteration indices similar to those found in other deserts of the world. The desert sands have been more influenced by sedimentary and granitic sources. This is supported by the fact that Ba and Sr concentration values of the desert sands are within the range of the Ba and Sr concentration values of the Colorado River quartz-rich sediments. The Sr values are also linked to the presence of Ca-bearing minerals. The Zr values are linked to the sedimentary sources and heavy mineral content in the desert dunes. The Golfo de Santa Clara and Puerto Peñasco coastal dune sands are influenced by long shore drift, tidal and aeolian processes. Coarse grains are found on the flanks whereas fine grains are on the crest of the dunes. High tidal regimens, long shore drift and supply from Colorado Delta River sediments produce quartz-rich sands on the beach that are subsequently transported into the coastal dunes. Outcrops of Quaternary sedimentary rocks and granitic sources increase the sedimentary and plutonic lithic content of the coastal dune sands. The chemical index of alteration (CIA) values for the desert and coastal dune sands indicate that both dune types are chemically homogeneous. The trace element values for the coastal dune sands are similar to those found for the desert dune sands. However, an increase in Sr content in the coastal dune sands may be due to more CaCO3 of biogenic origin as compared to the desert dune sands. Correlations between the studied parameters show that the dune sands are controlled by sedimentary sources (e.g. Colorado River Delta sediments), since heavy minerals are present in low percentages in the dune sands, probably due to little heavy mineral content from the source sediment; grain sizes in the dune sands are coarser than those in which heavy minerals are found and/or the wind speed might not exert a potential entrainment effect on the heavy mineral fractions to be transported into the dune. A cluster analysis shows that the El Pinacate group is significantly different from the rest of the dune sands in terms of the grain-size parameters due to longer transport of the sands and the long distance from the source sediment, whereas the Puerto Peñasco coastal dune sands are different from the rest of the groups in terms of their geochemistry, probably caused by their high CaCO3 content and slight decrease in the CIA value. Copyright © 2006 John Wiley & Sons, Ltd. [source]

Bed morphology and generation of step,pool channels

EARTH SURFACE PROCESSES AND LANDFORMS, Issue 11 2008
Roman B. Weichert
Abstract Flume experiments have been carried out to study the formation processes and the bed morphology of step,pool channels. From the experiments different step types and step configurations could be distinguished depending on the stream power. These step types can be seen as an image of the generation mechanisms of step,pool systems. These results suggest that the bed roughness geometry develops towards a condition that provides the maximum possible bed stability for a given grain size distribution. In contrast to a variety of other studies, antidunes did not contribute to the generation of the step structures. However, the data of the presented study fits well into the region of antidune formation proposed by Kennedy for sand-bed rivers. This observation points out that step,pool field-data located in the Kennedy region do not inevitably prove that antidunes played a role in step development. It is rather proposed that in Kennedy's region of antidune formation there exist hydraulic conditions where the flow resistance is maximized. It is suggested that such maximum flow resistance is associated with an optimal distance between the bedforms and their height, independently of whether these are antidunes in sand- and gravel-bed rivers or step,pool units in boulder-bed streams. The considerations of the Kennedy region of antidune formation and the analysis of planform step types depending on stream power both suggest that steep channels have a potential for self-stabilization by modifying the step,pool structure towards a geometry that provides maximum flow resistance and maximum bed stability. Copyright © 2008 John Wiley & Sons, Ltd. [source]

Interactions between fauna and sediment control the breakdown of plant matter in river sediments

FRESHWATER BIOLOGY, Issue 4 2010
SIMON NAVEL
Summary 1. A substantial portion of particulate organic matter (POM) is stored in the sediment of rivers and streams. Leaf litter breakdown as an ecosystem process mediated by microorganisms and invertebrates is well documented in surface waters. In contrast, this process and especially the implication for invertebrates in subsurface environments remain poorly studied. 2. In the hyporheic zone, sediment grain size distribution exerts a strong influence on hydrodynamics and habitability for invertebrates. We expected that the influence of shredders on organic matter breakdown in river sediments would be influenced strongly by the physical structure of the interstitial habitat. 3. To test this hypothesis, the influence of gammarids (shredders commonly encountered in the hyporheos) on degradation of buried leaf litter was measured in experimental systems (slow filtration columns). We manipulated the structure of the sedimentary habitat by addition of sand to a gravel-based sediment column to reproduce three conditions of accessible pore volume. Ten gammarids were introduced in columns together with litter bags containing alder leaves at a depth of 8 cm in sediment. Leaves were collected after 28 days to determine leaf mass loss and associated microbial activity (fungal biomass, bacterial abundance and glucosidase, xylosidase and aminopeptidase activities). 4. As predicted, the consumption of buried leaf litter by shredders was strongly influenced by the sediment structure. Effective porosity of 35% and 25% allowed the access to buried leaf litter for gammarids, whereas a lower porosity (12%) did not. As a consequence, leaf litter breakdown rates in columns with 35% and 25% effective porosity were twice as high as in the 12% condition. Microbial activity was poorly stimulated by gammarids, suggesting a low microbial contribution to leaf mass loss and a direct effect of gammarids through feeding activity. 5. Our results show that breakdown of POM in subsurface waters depends on the accessibility of food patches to shredders. [source]

Existing in plenty: abundance, biomass and diversity of ciliates and meiofauna in small streams

FRESHWATER BIOLOGY, Issue 4 2008
JULIA REISS
Summary 1. The ciliate and metazoan meiofaunal assemblages of two contrasting lowland streams in south-east England were examined over the period of a year, using a high taxonomic resolution. Monthly samples were taken from an oligotrophic, acid stream (Lone Oak) and a circumneutral, nutrient-rich stream (Pant) between March 2003 and February 2004. 2. We assessed the relative importance of ciliates and rotifers within the small-sized benthic assemblage with respect to their abundance, biomass and species richness. In addition, we examined the influence of abiotic and biotic parameters and season on the assemblage composition at two levels of taxonomic resolution (species and groups). 3. Ciliates dominated the assemblages numerically, with maximum densities of over 900 000 and 6 000 000 ind. m,2 in Lone Oak and Pant respectively. Rotifers and nematodes dominated meiofaunal densities, although their contribution to total meiofaunal biomass (maxima of 71.9 mgC m,2 in Lone Oak and of 646.8 mgC m,2 in the Pant) was low and rotifer biomass equalled that of ciliates. 4. Although the two streams differed in terms of total abundance of ciliates and meiofauna and shared only 7% of species, the relative proportion of groups was similar. Sediment grain size distribution (the percentile representing the 0.5,1 mm fraction) was correlated with assemblage structure at the species level, revealing the tight coupling between these small organisms and their physical environment. Seasonal changes in the relative abundance of groups followed similar patterns in both streams, and were correlated with the abundance of cyclopoid copepods and temperature. 5. Information on these highly abundant but often overlooked faunal groups is essential for estimates of overall abundance, biomass, species richness and productivity in the benthos, and as such has important implications for several areas of aquatic research, e.g. for those dealing with trophic dynamics. [source]

A study of Holocene floodplain particle size characteristics with special reference to palaeochannel infills from the upper Severn basin, Wales, UK

GEOLOGICAL JOURNAL, Issue 2 2001
Mark Patrick Taylor
Abstract Multiple sedimentary units from floodplain reaches at Welshpool on the upper River Severn and at the confluence of the Afon Tanat and Afon Vyrnwy (mid-Wales, UK) were examined to ascertain if they have distinctive particle size characteristics. Changes in particle size characteristics and their possible relationship to known human and climatic impacts are also discussed. Ellipse plots of particle size characteristics from the River Severn floodplain at Welshpool show that coarse-grained outwash deposits can be clearly discriminated from channel margin or palaeochannel sediments. In contrast, at the Afon Tanat,Vyrnwy study reach, this discrimination is not seen so clearly. The relationships between age and particle size characteristics from the most sampled sedimentary environment, palaeochannel infills, were also examined. The data from the River Severn floodplain at Welshpool show that palaeochannel sediments reveal a gradual but clear increase in particle size from the mid- to late Holocene towards the present day. Sediments deposited in the period 90,160 years BP are markedly coarser. It is suggested that these changes may be related to the combined effect of land-use changes, metal mining impacts and changes in flood frequency and magnitude that occurred at this time within the upper Severn basin. In contrast, the particle size characteristics of post Late Devensian/Early Holocene units from Tanat,Vyrnwy palaeochannels were random with no discernible age,size patterns. It is suggested that the non-systematic grain size distribution may be due to the steeper valley gradients of the Tanat,Vyrnwy system (and by inference higher stream powers) and its relatively narrow valley form enabling more effective coupling between coarser outwash deposits found on and at the edges of hillslopes and the valley floor. Although the two study reaches have undergone comparable environmental change during the Holocene and lie in the piedmont zone of their catchments, palaeochannel units of the same age possess distinctly different characteristics. Intrinsic reach-scale geomorphic factors would appear to preclude the uniform application of particle size characteristics to determine alluvial response to environmental change. Consequently, care needs to be applied to the use of such data for environmental discrimination because the phenomenon of equifinality means that a specific set of sediment characteristics is not necessarily exclusive to specific fluvial environments in either space or time. Copyright © 2001 John Wiley & Sons, Ltd. [source]

The triggering of debris flow due to channel-bed failure in some alpine headwater basins of the Dolomites: analyses of critical runoff

HYDROLOGICAL PROCESSES, Issue 13 2008
C. Gregoretti
Abstract The debris deposits at the bottom of very steep natural channels and streams in high mountain areas can be mobilized by runoff, triggering a water,sediment mixture flow known as debris flow. The routing of debris flow through human settlements can cause damage to civil structures and loss of human lives. The prediction of such an event, or the runoff discharge that triggers it, assumes an interest in risk analyses and the planning of defence measures. The object of this study is to find a method to determine the critical runoff value that triggers debris flow as a result of channel-bed failure. Historical and rainfall data on 30 debris flows that occurred in six watersheds of the Dolomites (north-eastern Italian Alps) were collected from different sources. Field investigations at the six sites, together with the hydrologic response to the rainfalls that triggered the events, were performed to obtain a realistic scenario of the formation of the debris flow there occurred. Field observations include a survey along the channel of the triggering reach of debris flow, with measurements of the channel slope and cross-section and sampling of debris deposits for grain size distribution. Simulated runoff discharge values based on the rainfall recorded by pluviometers were then compared with values obtained through experimental criteria on the initiation and formation of debris flow by bed failure. The results are discussed to provide a plausible physical-based method for the prediction of the triggering of debris flow by channel-bed failure. Copyright © 2007 John Wiley & Sons, Ltd. [source]

Sediment infiltration traps: their use to monitor salmonid spawning habitat in headwater tributaries of the Cascapédia River, Québec

HYDROLOGICAL PROCESSES, Issue 20 2005
André E. Zimmermann
Abstract Sediment infiltration can clog salmon nests and reduce egg survival. As a countermeasure, environmental managers often deploy infiltration traps to monitor sediment infiltration. Traps provide a repeatable means of measuring infiltration and enable comparisons to be made between sites. Results from infiltration rates measured with traps have also been used to estimate infilling rates into salmon nests. Application of these data is questionable, as the composition of the bed and the amount of fine sediment within the bed is known to affect infiltration rates. Thus, infiltration rates measured with infiltration traps may differ from the infiltration rates occurring in redd and riffle gravels. To examine how relationships between sediment infiltration rates varied between four watersheds, we continuously monitored suspended sediment transport, shear stress and infiltration rates at four sites over 5 months. We also compared infiltration rates measured with infiltration traps with changes in the hydraulic conductivity and subsurface grain size distribution of adjacent artificially constructed salmon nests and natural riffle gravels. Among the four watersheds, clear differences in sediment infiltration rates were observed. The differences correlated with the subsurface silt content but no strong relationship existed between land-use or basin physiography/geology. Despite observing an average of 30 kg m,2 of sediment finer than 2 mm being deposited in the infiltration traps during the study, no change in redd or riffle substrate was observed. If the deposition rates measured with the traps reflect the processes in redds, enough sediment would have been deposited to inhibit egg emergence. However, no reduction in egg survival to the eyed stage was observed. In summary, our results show that infiltration traps with clean gravels can be used to detect intersite differences in sediment transport regimes. Extrapolations of sediment infiltration rates measured with such collectors to estimate infiltration rates in redds or riffles is, however, flawed. Copyright © 2005 John Wiley & Sons, Ltd. [source]

An enhanced constitutive model for crushable granular materials

INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 6 2010
Ali Daouadji
Abstract Studies in the past have tried to reproduce the mechanical behaviour of granular materials by proposing constitutive relations based on a common assumption that model parameters and parameters describing the properties, including gradation of individual grains are inevitably linked. However successful these models have proved to be, they cannot account for the changes in granular assembly behaviour if the grains start to break during mechanical loading. This paper proposes to analyse the relation between grading change and the mechanical behaviour of granular assembly. A way to model the influence of grain breakage is to use a critical state-based model. The influence of the amount of grain breakage during loading, depending on the individual grain strength and size distribution, can be introduced into constitutive relations by means of a new parameter that controls the evolution of critical state with changes in grain size distribution. Experimental data from a calcareous sand, a quartz sand, and a rockfill material were compared with numerical results and good-quality simulations were obtained. The main consequences of grain breakage are increased compressibility and a gradual dilatancy disappearance in the granular material. The critical state concept is also enriched by considering its overall relation to the evolution of the granular material. Copyright © 2009 John Wiley & Sons, Ltd. [source]

Strain rates from snowball garnet

JOURNAL OF METAMORPHIC GEOLOGY, Issue 3 2003
C. Biermeier
Abstract Spiral inclusion trails in garnet porphyroblasts are likely to have formed due to simultaneous growth and rotation of the crystals, during syn-metamorphic deformation. Thus, they contain information on the strain rate of the rock. Strain rates may be interpreted from such inclusion trails if two functions are known: (1) The relationship between rotation rate and shear strain rate; (2) the growth rate of the crystal. We have investigated details of both functions using a garnetiferous mica schist from the eastern European Alps as an example. The rotation rate of garnet porphyroblasts was determined using finite element modelling of the geometrical arrangement of the crystals in the rock. The growth rate of the porphyroblasts was determined by using the major and trace element distributions in garnet crystals, thermodynamic pseudosections and information on the grain size distribution. For the largest porphyroblast size fraction (size L=12 mm) we constrain a growth interval between 540 and 590 °C during the prograde evolution of the rock. Assuming a reasonable heating rate and using the angular geometry of the spiral inclusion trails we are able to suggest that the mean strain rate during crystal growth was of the order of =6.6 × 10,14 s,1. These estimates are consistent with independent estimates for the strain rates during the evolution of this part of the Alpine orogen. [source]

Effect of chemical heterogeneity on adsorbed solute dispersion at column scale

AICHE JOURNAL, Issue 4 2008
Safia Semra
Abstract Chemical heterogeneity seems to be responsible for spreading increase of adsorbed solute breakthrough curves. Adsorption in fixed beds assumes chemically homogeneous media. However, this is not always true, in particular when natural sands or mixed adsorbent filters are used in drinking water purification. Neglecting eventual effect of chemical heterogeneity may engender false modeling bases. So, considering homogeneous grain size distribution, the effect of chemical heterogeneity on global dispersion in porous media has been investigated experimentally in this article at column scale. Breakthroughs of adsorbed solute showed a visible effect of chemical heterogeneity on solute global dispersion increasing. The more heterogeneous the medium, the more spread the adsorbed solute breakthrough. Reduced variance showed a linear variation with the chemical heterogeneity scale at closely constant media global capacity. A pseudo-homogeneous model has been developed to simulate experimental data by increasing dispersion parameter. © 2008 American Institute of Chemical Engineers AIChE J, 2008 [source]

Pulsed Electric Current Sintering of Silicon Nitride

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2003
Motohiro Suganuma
Pulsed electric current sintering (PECS) has been used to densify ,-Si3N4 powder doped with oxide additives of Y2O3 and Al2O3. A full density (>99%) was achieved with virtually no transformation to ,-phase, resulting in a microstructure with fine equiaxed grains. With further holding at the sintering temperature, the ,-to-, phase transformation took place, concurrent with an exaggerated grain growth of a limited number of elongated ,-grains in a fine-grained matrix, leading to a distinct bimodal grain size distribution. The average grain size was found to obey a cubic growth law, indicating that the growth is diffusion-controlled. In contrast, the densification by hot pressing was accompanied by a significant degree of the phase transformation, and the subsequent grain growth gave a broad normal size distribution. The apparent activation energy for the phase transformation was as high as 1000 kJ/mol for PECS, almost twice the value for hot pressing (,500 kJ/mol), thereby causing the retention of ,-phase during the densification by PECS. [source]

Coarsening of Faceted Crystals

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2002
Gregory S. Rohrer
The influence of the nucleation energy barrier on the capillary-driven coarsening of faceted crystals that exchange material by diffusion is quantified. Our calculations are based on the assumption that the transport of material between particles must happen in series with the nucleation of partial layers on flat facets. Using a numerical model based on this idea, we simulate the time evolution of distributions of crystals that are made up of perfect faceted crystals (without step-producing defects), crystals containing step-producing defects, and mixtures of the two types. We find that the coarsening of a distribution containing only perfect faceted crystals is arrested at a size where the nucleation energy barrier becomes prohibitive. This critical size ranges from a few nanometers to several hundred nanometers, depending on material parameters and experimental conditions. When a small fraction of the crystals have step-producing defects (for these crystals the nucleation energy barrier vanishes), they can grow to large sizes at the expense of the perfect crystals and a bimodal grain size distribution is created. Based on these results, we hypothesize that when abnormal coarsening is observed in nature, it results from the presence of a small number of crystals with step-producing defects. [source]

Experiments on sediment trap efficiency in reservoirs

LAKES & RESERVOIRS: RESEARCH AND MANAGEMENT, Issue 1 2005
Horacio Toniolo
Abstract Sediment trap efficiency plays a key role in the effective operational life of reservoirs. This paper presents the results of five laboratory experiments on trap efficiency. An over-spilling condition and four gaps located at the downstream end of a reservoir were analysed in this study. The experimental design assumed a river carrying two phases of sediment flowing into a one-dimensional reservoir. The coarse sediment (sand) was deposited and formed a defined prograding delta. The fine sediment (mud) formed a dilute suspension of wash load in the river. As the river entered the reservoir, the muddy water plunged on the foreset, forming a turbidity current. The turbidity current deposits, in turn, formed a bottomset. Black coal slag and white glass beads were used to simulate sand and mud. Their specific densities were 2.6 and 2.5 for black coal and beads, respectively. The water surface elevation in the reservoir was approximately similar in all experiments. Neither the water nor sediment discharge conditions were changed during the experimental runs. Suspended sediment was sampled through seven siphons; six of these being components of a rake in which they were vertically stacked. The last siphon was positioned on the rake outlet. Sediment samples were taken three times, at approximately evenly spaced intervals in the experiments. Suspended sediment concentration and grain size distribution were calculated for each siphon. The bed sediment deposit was sampled after each experiment. Sediment trap efficiency in the reservoir was calculated. Experimental results show the maximum venting capacity (minimum trap efficiency) occurs under over-spilling conditions. [source]

Synthesis of ultrafine titanium carbonitride powders,

APPLIED ORGANOMETALLIC CHEMISTRY, Issue 5 2001
Frederic Monteverde
Abstract Titanium-carbonitride-based materials are very hard materials with increasing technical importance. They are mainly used in composites with various metal carbides and/or metallic binders (cermets) for metal cutting operations. These applications call for the synthesis of titanium carbonitride powders with homogeneous chemical composition, as small as possible grain size and narrower grain size distribution. Nowadays on the market, only commercial submicrometric (0.5,2,,m) powders are available. Starting from blends of nanosize commercial TiN or TiO2 powders mixed with different carbon powders (carbon black, active carbon), this study aimed to set up a low-cost process to synthesize fine and pure TiC1,X,NX powders with an X value close to 0.5. The morphology of the as-obtained powders and the progress of the reaction were investigated by scanning electron microscopy and X-ray diffraction. The stoichiometric parameter X was estimated on the basis of a TiC1,X,NX Raoultian solid solution together with Vegard's rule. The results are presented and discussed to assess relations between powder characteristics and processing conditions. The most encouraging results were obtained using a mixture TiN,+,10,wt%C (carbon black) processed at 1430,°C for 3,h under flowing argon. Regularly shaped particles with limited agglomeration ranged from 100 to 300,nm and an X value close to 0.5 Copyright © 2001 John Wiley & Sons, Ltd. [source]

The physical scale modelling of braided alluvial architecture and estimation of subsurface permeability

BASIN RESEARCH, Issue 3 2002
D. J. Moreton
ABSTRACT The quantitative modelling of fluvial reservoirs, especially in the stages of enhanced oil recovery, requires detailed three-dimensional data at both the scale of the channel belt and within-channel. Although studies from core, analogue outcrop and modern environments may partially meet these needs, they often cannot provide detail on the smaller-scale (i.e. channel-scale) heterogeneity, frequently suffer from limited three-dimensional exposure and cannot be used to examine the influence of different variables on the process,deposit relationship. Physical modelling offers a complementary technique that can address many of these quantitative requirements and holds great future potential for integration with reservoir modelling. Physical modelling provides the potential to upscale results and derive reservoir information on three-dimensional facies geometry, connectivity and permeability. This paper describes the development and use of physical modelling, which employs generic Froude-scaling principles, in an experimental basin that permits aggradation in order to model the morphology and subsurface depositional stratigraphy of coarse-grained braided rivers. An example is presented of a 1:50 scale model based on the braided Ashburton River, Canterbury Plains, New Zealand and the adjacent late Quaternary braided alluvium exposed in the coastal cliffs. Critically, a full, bimodal grain size distribution (20% sand and 80% gravel) was used to replicate the prototype, which allows the realistic reproduction of the surface morphology and importantly permits grain size sorting during deposition. Uncertainties associated with the compression of time, sediment mass balance and the hydrodynamics of the finest particle sizes do not appear to affect the reproducibility of stratigraphy between experimental and natural environments. Sectioning of the preserved sedimentary sequence in the physical model allows quantification of the geometry, shape, spatial distribution and internal sedimentary structure of the coarse- and fine-grained facies. A six-fold facies scheme is proposed for the model braided alluvium and a direct link is established between the grain size distribution and facies type: this allows permeability to be estimated for each facies, which can be mapped onto two-dimensional vertical cross-sections of the preserved stratigraphy. Results demonstrate the dominance of four facies based on permeability that range over three orders of magnitude in hydraulic conductivity. Quantification of such variability, and linkage to both vertical proportion curves for facies distribution and connectivity presents significant advantages over other methodologies and offers great potential for the modelling of heterogeneous braided river sediments at the within channel-belt scale. This paper outlines how physical models may be used to develop high-resolution, geologically-accurate, object-based reservoir simulation models. [source]

Storegga tsunami sand in peat below the Tapes beach ridge at Harøy, western Norway, and its possible relation to an early Stone Age settlement

BOREAS, Issue 3 2003
STEIN BONDEVIK
One of the early problems with the Storegga tsunami deposit was how to distinguish it from deposits of the midHolocene (Tapes) transgression. An excavation on Harøy, an island on the outermost western coast of Norway, shows a distinct, clean sand bed embedded in peat and clearly separated from the overlying Tapes beach deposits. This sand bed continues in the peat landwards of the beach ridge for at least 60 m. Radiocarbon dates of the peat show that the sand was deposited some time between 6900 and 7700 yr BP. The sedimentary structures of the bed, the 14C dates, and the fact that this is the only sand bed in the peat, suggest that the sand bed was deposited by a short-lived event, the Storegga tsunami. On the neighbouring island, Fjørtoft, a Stone Age settlement, dated to 7500 yr BP, was discovered in the early 1970s. The settlement was found underneath a sand bed that later had been covered by the Tapes beach ridge deposits. When discovered, the sand covering the settlement was inferred as eolian sand. However, this investigation shows that the Storegga tsunami deposited a widespread sand bed on the land surface around this time with a similar grain size distribution to eolian sand. It is therefore suggested that the sand bed covering this settlement was deposited from the Storegga tsunami. Both the stratigraphy and 14C dates demonstrate that the Tapes transgression maximum was reached well after the Storegga tsunami on Harøy, between 6500 and 6100 yr BP. [source]

Sedimentological, modal analysis and geochemical studies of desert and coastal dunes, Altar Desert, NW Mexico

Formation of the complex linear dunes in the central Taklimakan Sand Sea, China

EARTH SURFACE PROCESSES AND LANDFORMS, Issue 6 2004
Xunming Wang
Abstract The formation of the complex linear dunes in the central Taklimakan Sand Sea is discussed based on analyses of wind regimes, sand grain size distributions on the topography of the dunes, and a combination of geomorphic and geophysical investigations into the morphology of the dunes. Complex linear dune formation is shown to have ,ve stages. Analysis clearly shows that under the control of wind regime, sand supply and other factors, the simple linear dunes move sideways while they evolve. This is the main cause for the formation of complex linear dunes in the central Taklimakan Sand Sea. We have not collected enough evidence to show whether the complexity of the complex linear dunes is left over from previous wind regimes or whether the previous wind regimes had different dominant wind directions compared to those of modern winds. The evolutionary processes of complex linear dunes in the region partly support the theory of ,barchan evolution' but do not support the ,roll-vortex' and ,bimodal wind regime' hypotheses. After the complex linear dunes were developed, the local wind regime and the other controls such as sand supply suggest it is possible for them to maintain their linear shape. The evolutionary process discussed is limited to the region indicated in this paper, and may not be applicable to the whole Taklimakan Sand Sea. There are different evolutionary processes in different dune,elds because of variations in the factors that control complex linear dune formation. Copyright © 2004 John Wiley & Sons, Ltd. [source]

Structural patterns in coarse gravelriver beds: typology, survey and assessment of the roles of grain size and river regime

GEOGRAFISKA ANNALER SERIES A: PHYSICAL GEOGRAPHY, Issue 1 2002
Lea Wittenberg
The concept of river-bed stability as indexed by the occurrence of stable bed forms was examined in humid-temperate perennial streams and in Mediterranean ephemeral streams. The study examined the structural patterns of bed forms and their spatial distribution between temperate-humid and Mediterranean streams. Study sites in Northumberland, UK, and Mt. Carmel, Israel, were selected for their morphometric similarity, despite the contrast in climate, vegetation and hydrological regime. Fieldwork was based on a large number of Wolman grain size distributions and structure measurements along cross-sections at seven sites; Differences in mean grain size of bed structures were estimated using the general linear model (GLM) procedure and Duncan's multiple range test. Based on field evidence, river-bed configurations were divided into structural categories, according to the depositional setting of each measured particle on the river bed. Statistical analysis confirmed former qualitative descriptions of small-scale bed forms. The study identified spatial segregation in bed form distribution. In general, 30,40%of the bed material in the surveyed perennial streams was clustered, in contrast to approximately 10%in the ephemeral counterparts. The sorting index revealed higher values for the perennial streams, namely 2.39,3.59 compared with 1.73,2.07 for the ephemeral counterparts. It is suggested that the degree of sediment sorting and the proportion of clusters are strongly related. Sediment sorting, sediment supply and the hydrological regime explain the mechanism of cluster formation. It is assumed that climate shifts or human interference within river basins might affect the regional characteristic flood hydrograph, and consequently alter the sedimentary character of the river bed. In the case where river bed stability is reduced owing to changes in cluster bed form distribution, rivers that normally do not yield a significant amount of sediment might be subject to notable sedimentation problems. [source]

A Bayesian approach to inverse modelling of stratigraphy, part 1: method

BASIN RESEARCH, Issue 1 2009
Karl Charvin
ABSTRACT The inference of ancient environmental conditions from their preserved response in the sedimentary record still remains an outstanding issue in stratigraphy. Since the 1970s, conceptual stratigraphic models (e.g. sequence stratigraphy) based on the underlying assumption that accommodation space is the critical control on stratigraphic architecture have been widely used. Although these methods considered more recently other possible parameters such as sediment supply and transport efficiency, they still lack in taking into account the full range of possible parameters, processes, and their complex interactions that control stratigraphic architecture. In this contribution, we present a new quantitative method for the inference of key environmental parameters (specifically sediment supply and relative sea level) that control stratigraphy. The approach combines a fully non-linear inversion scheme with a ,process,response' forward model of stratigraphy. We formulate the inverse problem using a Bayesian framework in order to sample the full range of possible solutions and explicitly build in prior geological knowledge. Our methodology combines Reversible Jump Markov chain Monte Carlo and Simulated Tempering algorithms which are able to deal with variable-dimensional inverse problems and multi-modal posterior probability distributions, respectively. The inverse scheme has been linked to a forward stratigraphic model, BARSIM (developed by Joep Storms, University of Delft), which simulates shallow-marine wave/storm-dominated systems over geological timescales. This link requires the construction of a likelihood function to quantify the agreement between simulated and observed data of different types (e.g. sediment age and thickness, grain size distributions). The technique has been tested and validated with synthetic data, in which all the parameters are specified to produce a ,perfect' simulation, although we add noise to these synthetic data for subsequent testing of the inverse modelling approach. These tests addressed convergence and computational-overhead issues, and highlight the robustness of the inverse scheme, which is able to assess the full range of uncertainties on the inferred environmental parameters and facies distributions. [source]