internal structure of aeolian dunes in abu dhabi determined using ground penetrating radar a ground penetrating radar survey of aeolian dunes in the al liwa area of abu dhabi reveals a variety of dipping reflectors which are interpreted as primary sedimentary structures the interpretation of the radar profiles has been confirmed by bulldozing trenches through the study area and comparing logged sections in the trenches with the radar profiles nnw sse orientated radar profiles approximately parallel to the prevailing wind direction show two sets of dipping reflectors which are interpreted as sets of cross stratification and second and third order bounding surfaces radar profiles orientated wsw ene across the prevailing wind direction are dominated by concave up reflectors which are interpreted as trough shaped scours and sets of trough cross stratification produced by oblique progradation of barchanoid dunes nested troughs with small sets of trough cross stratification within larger troughs may be due to reactivation following wind reversal or the superposition of small dunes on larger dunes and the fill of large dune troughs by smaller dunes convex upwards reflectors are interpreted as linear spurs on the convex portions of sinuous dunes or erosional remnants between troughs overall there is a tendency for the larger second order bounding surfaces to dip downwind which confirms brookfield s ideas of the relative migration paths of dunes and draa

the sedimentary structure of linear sand dunes linear sand dunes dunes that extend parallel to each other rather than in star like or crescentic forms are the most abundant type of desert sand dune1 but because their development and their internal structure are poorly understood they are rarely recognized in the rock record2 models of linear dune development2 3 4 5 6 have not been able to take into account the sub surface structure of existing dunes but have relied instead either on the extrapolation of short term measurements of winds and sediment transport or on observations of near surface internal sedimentary structures from such studies it has not been clear if linear dunes can migrate laterally2 7 8 here we present images produced by ground penetrating radar showing the three dimensional sedimentary structure of a linear dune in the namib sand sea where some of the world s largest linear dunes are situated these profiles show clear evidence for lateral migration in a linear dune moreover the migration of a sinuous crest line along the dune produces divergent sets of cross stratification which can become stacked as the dune height increases and large linear dunes can support superimposed dunes that produce stacked sets of trough cross stratification these clear structural signatures of linear dunes should facilitate their recognition in geological records

morphodynamics of barchan and transverse dunes using a cellular automaton model 1 we analyze the morphodynamics of bedforms produced under unidirectional flow conditions by a lattice gas cellular automaton designed to study sediment transport in this model the same instability is responsible for the formation of dunes on flat sand beds and the initiation of superimposed bedforms on dune slopes in transverse dune fields secondary bedforms increase crestline sinuosity and number of defects end of crestlines on the other hand avalanches and lateral grain motions tend to eliminate these defects to produce more regular crestlines lateral fluxes of sediment are also essential for stabilizing the shape of isolated barchan dunes we measure the propagation speed of superimposed bedforms on steady state barchan dunes and show how they contribute to the formation and detachment of smaller barchans along horns the model predicts that barchan dunes are not scale invariant and that their shape varies with respect to the strength of the flow in addition we show that the increase in bed shear stress between the ground and the crest is proportional to the dune aspect ratio finally we present a general methodology for estimating the sediment flux over the brink from dune aspect ratio and flow velocity using these fluxes we rescale the propagation speed of different generations of bedforms to verify that dunes and secondary bedforms are dynamically identical despite more fluctuations than in the case of isolated barchan dunes all these geometric and dynamical relationships hold for a population of dunes with complex dune dune interactions

factors determining desert dune type while most observers recognize four elemental types of desert dunes longitudinal transverse barchan and star1 3 there is little agreement about which factors determine these types the angular relationships between the resultant of sand shifting winds and both the crest and principal slipfaces of the elemental types have been discussed qualitatively for many decades these relationships have been quantified but the wide range of dune types that combine the elemental forms and the juxtaposition of different dune types show that wind regime is not the only determinant for example sand availability has been considered important we have now quantified the importance of both wind regime and sand availability as determinants of dune type we show here that the four elemental types occur in areas uniquely defined by both the equivalent sand thickness of sediments held in the dunes and a measure of directional variability of sand moving winds wind strength does not appear to be important vegetation has an ambiguous role and the particle size of dune sediments is unimportant

is there evidence for global scale forcing of southern hemisphere quaternary desert dune accumulation a quantitative method for testing hypotheses of dune system development luminescence dating of desert dune sediments has generated many hundreds of ages many used in reconstructions of quaternary environmental changes others in attempts to elucidate dune processes environmental and climatic interpretations of these records have proved problematic and it remains challenging to test hypotheses of the systematic response of dunefields to changes in external forcing in the past and to make predictions of the future we use a method that quantifies dune sediment accumulation to interpret dune luminescence age datasets rather than simply using the ages themselves as proxies of change the accumulation intensity method allows periods of dune sediment accumulation here over the timescale 102 105 years to be identified from compilations of dated sand sea stratigraphic sequences we apply this approach to two of the largest dune age datasets from southern africa and australia testing whether or not dunefield accumulation has co varied in the late quaternary and whether systematic relationships to external drivers at global hemispheric regional and local scales can be identified

the formation of shadow dunes abstract an investigation of the modification of airflow by a discrete semi circular roughness element in this case vegetation the resulting flow structure and the formation of pyramidal shaped shadow dunes is presented two erect pioneer sand dune grasses festuca littoralis and ammophila arenaria were used in the experiments which were conducted in both the field and a wind tunnel for a range of wind velocities 4 20 m sec 1 the three dimensional flow structure about the vegetation is dominated by horizontal separation characterized by symmetrically opposed reversing vortices which form within a triangular wake a vertical separation envelope is also formed over the vegetation pyramidal shadow dunes are formed within this flow structure when aeolian sand transport occurs shadow dune height is dependent on element basal width and repose angle of the sand shadow dune length is dependent on element basal width and wind velocity the results are extrapolated to include all semi porous to non porous semicircular discrete roughness elements the role of shadow dune formation in the development of foredunes and related environments is examined

modelling vegetated dune landscapes this letter presents a self organising cellular automaton model capable of simulating the evolution of vegetated dunes with multiple types of plant response in the environment it can successfully replicate hairpin or long walled parabolic dunes with trailing ridges as well as nebkha dunes with distinctive deposition tails quantification of simulated landscapes with eco geomorphic state variables and subsequent cluster analysis and pca yields a phase diagram of different types of coastal dunes developing from blow outs as a function of vegetation vitality this diagram indicates the potential sensitivity of dormant dune fields to reactivation under declining vegetation vitality e g due to climatic changes nebkha simulations with different grid resolutions demonstrate that the interaction between the abiotic geomorphic processes and the biological vegetation component life introduces a characteristic length scale on the resultant landforms that breaks the typical self similar scaling of un vegetated bare sand dunes

origin and lateral migration of linear dunes in the qaidam basin of nw china revealed by dune sediments internal structures and optically stimulated luminescence ages with implications for linear dunes on titan linear dunes which are widespread on earth and on some other planets are generally considered to have a depositional origin and to form by loose sand under a bidirectional wind regime or by stabilized sand under a unidirectional wind regime while the wind rift erosional origin model has commonly been rejected due largely to lack of sufficient evidence and difficulty in morphodynamic interpretations the linear dunes in the qaidam basin have been proposed to form as self extending lee dunes under a unidirectional wind regime owing to a high level of total silt clay and salt tscs content or cohesiveness of sediments and they have undergone southward lateral migration at rates of up to 3 m yr we present herein results from studies of the sediments internal structures and optically stimulated luminescence osl ages to show that 1 the linear dunes in the qaidam basin are most likely of erosional origin similar to yardangs with orientations controlled by strikes of joints 2 tscs content or related sediment cohesion is unlikely to be an important factor determining the formation of these linear dunes and 3 the linear dunes consist of 0 81 3 21 ka cross strata with small lateral migration rates of 0 02 0 04 m yr our findings evidently suggest that linear dunes can be of erosional origin we therefore propose that the possibility of erosional origin should not be ruled out for linear dunes in some other dune fields on earth as well as on titan if the control of tectonic structures on the orientation of wind eroded ridges is taken into account morphodynamic interpretations for the wind rift model may become much simpler

windy mars a dynamic planet as seen by the hirise camera 1 with a dynamic atmosphere and a large supply of particulate material the surface of mars is heavily influenced by wind driven or aeolian processes the high resolution imaging science experiment hirise camera on the mars reconnaissance orbiter mro provides a new view of martian geology with the ability to see decimeter size features current sand movement and evidence for recent bedform development is observed dunes and ripples generally exhibit complex surfaces down to the limits of resolution yardangs have diverse textures with some being massive at hirise scale others having horizontal and cross cutting layers of variable character and some exhibiting blocky and polygonal morphologies reticulate fine polygonal texture bedforms are ubiquitous in the thick mantle at the highest elevations

techniques to measure grain size distributions of loamy sediments a comparative study of ten instruments for wet analysis ten instruments for measuring the grain size distribution of loamy sediments in water were tested in a comparative study instruments tested were the malvern mastersizer s the coulter ls 200 the fritsch analysette 22 version c the horiba partica la 950 the sedigraph 5100 the atterberg cylinder the coulter multisizer 3 the cis 100 the eyetech and the image analysis software histolab the first four instruments use laser diffraction the sedigraph and atterberg cylinder are based on sedimentation the coulter multisizer 3 uses impedance measurements and the last three instruments use optical techniques grain size analyses were carried out on four sediments with median grain diameters of approximately 35 30 12 and 9 m respectively and with no particles 90 m the laser diffraction instruments produced the best results for the various criteria considered in this study followed by the coulter multisizer 3 electrical sensing zone technique the sedimentation techniques and the optical methods however the study also showed that there is no such thing as an ideal method because techniques may score differently according to the criterion tested an appropriate evaluation thus requires that various criteria be considered issues to be taken into account are type of sediment quantity of sediment available speed of measurement complexity of the measurement protocol processing of the data reproducibility of the results and the specific aims of the study undertaken

dynamic processes acting on a longitudinal seif sand dune abstract field measurements were made on a longitudinal dune in the sinai desert in order to understand its morphology and dynamics the field measurements contradicted the wind structure indicated by the helicoidal flow theory rather it was found that winds coming from two basically different directions at different times and striking the dune obliquely were responsible for sand transport and erosion or deposition along the lee flank the essence of this mechanism is the deflection of the wind airflow on the lee flank of the dune to a direction parallel to the crest line the occurrence of erosion or deposition depends upon the angle of incidence between the wind and the crest line when this angle is 40 the velocity of the deflected wind drops and deposition takes place on the lee flank the angle of incidence in each wind storm is changed intermittently between 30 and 100 along the dune because the dune meanders and because of the sinuous outline of the crest line in this manner sand transport and erosion or deposition occurs along the lee flank depending on the angle of incidence between the wind and the crest line as a result of the deflection of the wind the dune elongates at an average rate of more than 1 m per month peaks and saddles along the crest line advance at an average rate of 0 7 m per month the lack of uniformity in the effects of the wind on both sides of the dune creates a lack of uniformity in the rate of erosion and deposition this can explain the formation of peaks along the crest line of the dune

morphology and dynamics of star dunes from numerical modelling star dunes are common in sand seas but the mechanisms driving their formation are unclear numerical modelling indicates that the morphology of the dunes is controlled by the frequency of changes in the wind regime

sedimentary architecture of an ancient linear megadune barremian neuquen basin insights into the long term development and evolution of aeolian linear bedforms linear aeolian bedforms are the most abundant bedform type in modern earth sand seas and are very common in our solar system despite their abundance the long term development of these bedforms and its impact upon the resulting sedimentary architecture in the geological record is still poorly understood the aims of this paper are to study the exposed record of an ancient linear megadune in order to discuss the factors that impact the development and sedimentary architecture of aeolian linear bedforms the outcrops of the ancient troncoso sand sea barremian neuquen basin argentina provide a unique opportunity to access a preserved megadune record with an external body geometry that unequivocally confirms its linear morphology architectural analysis of the bedform record reveals significant differences for several aspects of cross stratified set bodies and bounding surfaces and allows for the identification of three architectural complexes insights from deterministic models analysis of set body relative chronology and distribution suggest that complexes result from distinctive phases in bedform development it also clearly shows that the megadune s construction was achieved by expansion from a core where the oldest deposits are located and that its development was characterized by sustained growth and strong longitudinal dynamics factors that impact the development and architecture of linear bedforms are identified discussed and compared to bedforms of transverse dynamics finally a scheme of expected styles of sedimentary architecture for linear bedforms is presented this case study shows how growth can be a critical factor conditioning linear bedform architecture and indicates how the preservation of certain styles of sedimentary architecture in the geological record may not be as unusual as previously thought

testing models of linear dune formation by provenance analysis with composite sediment fingerprints abstract the formative mechanisms of linear longitudinal dunes and dunefields remain uncertain and multiple hypotheses have been proposed a central debate is the degree to which dunes act as along dune sediment transport corridors implying that dunes grow primarily by extension or whether they are comprised of locally derived sands moved from adjacent interdunes the wind rift model sediment fingerprinting studies with origins in fluvial science have been shown to offer the possibility to trace the provenance of aeolian sands and thus elucidate transport pathways two models a monte carlo framework and a generalized likelihood uncertainty estimate framework are used here to provide quantitative estimates of the sediment sources that have supplied a linear dune in the central simpson desert of central australia four possible sources are identified that may have supplied the dune two adjacent interdunes one upwind low ridge of sand and a merging upwind dune two sites near the dune s crest are used as the target and provided twenty surface samples for analysis following geochemical assay stepwise discriminant function analysis identified optimum elemental sediment fingerprints for a variety of possible sediment pathway configurations results suggest that the sands of the dune are sourced predominantly from upwind dunes and sand sources and that likely contributions from neighbouring dune swales are typically the findings suggest recent sediment accretion of a linear dune dominated by along dune sand flux and thus support an extensional component for the development of such dunes whilst it is noted that at a point by point basis this might not exclude accretion by vertical growth as some have observed there is no clear support for a substantive contribution to the dune sands from adjacent interdunes moreover the use of contemporary sediment fingerprinting methods to question hypotheses of aeolian geomorphology suggests that such methods have great potential for addressing other terrestrial geomorphological questions where identifying sediment pathways can provide vital insight

geostatistical approaches for incorporating elevation into the spatial interpolation of rainfall this paper presents three multivariate geostatistical algorithms for incorporating a digital elevation model into the spatial prediction of rainfall simple kriging with varying local means kriging with an external drift and colocated cokriging the techniques are illustrated using annual and monthly rainfall observations measured at 36 climatic stations in a 5000 km 2 region of portugal cross validation is used to compare the prediction performances of the three geostatistical interpolation algorithms with the straightforward linear regression of rainfall against elevation and three univariate techniques the thiessen polygon inverse square distance and ordinary kriging larger prediction errors are obtained for the two algorithms inverse square distance thiessen polygon that ignore both the elevation and rainfall records at surrounding stations the three multivariate geostatistical algorithms outperform other interpolators in particular the linear regression which stresses the importance of accounting for spatially dependent rainfall observations in addition to the colocated elevation last ordinary kriging yields more accurate predictions than linear regression when the correlation between rainfall and elevation is moderate less than 0 75 in the case study q 2000 elsevier science b v all rights reserved

flume experiments on the alignment of transverse oblique and longitudinal dunes in directionally varying flows for more than a century geologists have wondered why some bedforms are orientated roughly transverse to flow whereas others are parallel or oblique to flow this problem of bedform alignment was studied experimentally using subaqueous dunes on a 3 6 m diameter sand covered turntable on the floor of a 4 m wide flume in each experiment two flow directions relative to the bed were produced by alternating the turntable between two orientations the turntable was held in each orientation for a short time relative to the reconstitution time of the bedforms the resulting bedforms were in equilibrium with the time averaged conditions of the bimodal flows dune alignment was studied for five divergence angles the angle between the two flow directions 45 67 5 90 112 5 and 135 the flow depth during all experiments was approximately 30 cm mean velocity was approximately 50 cm s 1 and mean grain diameter was 0 6 mm each experiment continued for 30 75 min during which time the flume flow was steady and the turntable position changed every 2 min at the end of each experiment water was slowly drained from the flume and dune alignment was measured transverse dunes defined relative to the resultant transport direction were created when the divergence angle was 45 and 67 5 and longitudinal dunes were created when the divergence angle was 135 at intermediate divergence angles dunes with both orientations were produced but transverse dunes were dominant at 90 and longitudinal dunes were dominant at 112 5 one experiment was conducted with a divergence angle of 135 and with unequal amounts of transport in the two flow directions this was achieved by changing the orientation of the turntable at unequal time intervals thereby causing the amount of transport to be unequal in the two directions the dunes formed during this experiment were oblique to the resultant transport direction these experimental dunes follow the same rule of alignment as wind ripples studied in previous turntable experiments in both sets of experiments the bedforms developed with the orientation having the maximum gross bedform normal transport the orientation at which the sum of the bedform normal components of the two transport vectors reaches its maximum value in other words the bedforms develop with an orientation that is as transverse as possible to the two flows in those cases where the two flows diverge by more than 90 and transport equal amounts of sand bedforms that are as transverse as possible to the two separate flows will be parallel to the resultant of the two flow vectors although such bedforms have been defined by previous work as longitudinal bedforms they are intrinsically the same kind of bedform as transverse bedforms

three dimensional reverse time migration of ground penetrating radar signals three dimensional 3d ground penetrating radar gpr systems and 3d seismic imaging techniques have been developing fast and evolving rapidly in the last decade ray based migration methods have been successfully applied to processing 3d gpr signals based on the similarity between electromagnetic and seismic waves however reverse time migration rtm of 3d gpr signals has not been well studied in the past in this paper we present a 3d rtm based on maxwell s equations for 3d gpr surveys migration recovers the true subsurface structure from a distorted and unfocused time profile and suppresses common electromagnetic clutter and noise rtm based on maxwell s equations can consider conductivity directly and compensate for the attenuation within a high conductivity zone compared with 2d rtm 3d rtm back propagates both in line and cross line signals simultaneously and can include complex 3d permittivity and conductivity models we have integrated a parallel finite difference time domain fdtd algorithm based on a hybrid mpi and openmp scheme to reduce the computational cost of 3d problems the 3d rtm experiments on an anomaly of em shape and a realistic sand dune model demonstrate the effective recovery of 3d subsurface structures

a decade of surface change on a namib linear dune survey data from return visits to a linear dune in the namib desert provide information about change in dune form over a decade the data demonstrate that although change on these large features is slow there is none the less considerable movement over this time scale the dunes are therefore confirmed to be currently active and not pleistocene relics

steady state saltation in air coupled equations of motion for steady state saltation over an infinite plane are derived and solved for a simplified model of the grain surface impact process experimentally observed features of the wind velocity profile in saltation are qualitatively reproduced including a diminution of the sub saltation layer mean wind speed as the friction speed increases in this model the surface impact velocity of the saltating grains remains relatively constant over a wide range of free stream shear stresses and the grain mass flux increases with friction speed uf less rapidly than uf3

reproducibility and utility of dune luminescence chronologies abstract optically stimulated luminescence osl dating of dune deposits has increasingly been used as a tool to investigate the response of aeolian systems to environmental change amalgamation of individual dune accumulation chronologies has been employed in order to distinguish regional from local geomorphic responses to change however advances in dating have produced chronologies of increasing complexity in particular questions regarding the interpretation of dune ages have been raised including over the most appropriate method to evaluate the significance of suites of osl ages when local noisy and discontinuous records are combined in this paper these issues are reviewed and the reproducibility of dune chronologies is assessed osl ages from two cores sampled from the same dune in the northeast rub al khali united arab emirates are presented and compared alongside an analysis of previously published dune ages dated to within the last 30 ka distinct periods of aeolian activity and preservation are identified which can be tied to regional climatic and environmental changes this case study is used to address fundamental questions that are persistently asked of dune dating studies including the appropriate spatial scale over which to infer environmental and climatic change based on dune chronologies whether chronological hiatuses can be interpreted how to most appropriately combine and display datasets and the relationship between geomorphic and palaeoclimatic signals chronological profiles reflect localised responses to environmental variability and climatic forcing and amalgamation of datasets with consideration of sampling resolution is required otherwise local factors are always likely to dominate using net accumulation rates to display ages may provide an informative approach of analysing and presenting dune osl chronologies less susceptible to biases resulting from insufficient sampling resolution

linear dunes on titan extensive dunes produced by wind transport of sand sized particles are common on earth and are also seen on mars and venus saturn9s moon titan is now shown to have similar dunes

seabed characterization on the new jersey middle and outer shelf correlatability and spatial variability of seafloor sediment properties abstract nearly 100 collocated grab samples and in situ 65 khz acoustic measurements were collected on the new jersey middle and outer shelf within an area that had previously been mapped with multibeam backscatter and bathymetry data and more recently with chirp seismic reflection profiling eighteen short cores were also collected and probed for resistivity based porosity measurements the combined data set provides a basis for empirically exploring the relationship among the remotely sensed data such as backscatter and reflection coefficients and directly measured seabed properties such as grain size distribution velocity attenuation and porosity we also investigate the spatial variability of these properties through semi variogram analysis to facilitate acoustic modeling of natural environmental variability grain size distributions on the new jersey shelf are commonly multi modal leading us to separately characterize coarse 4 mm fine

setting the length and time scales of a cellular automaton dune model from the analysis of superimposed bed forms 1 we present a new 3 d cellular automaton model for bed form dynamics in which individual physical processes such as erosion deposition and transport are implemented by nearest neighbor interactions and a time dependent stochastic process simultaneously a lattice gas cellular automaton model is used to compute the flow and quantify the bed shear stress on the topography local erosion rates are assumed to be proportional to the shear stress in such a way that there is a complete feedback mechanism between flow and bed form dynamics in the numerical simulations of dune fields we observe the formation and the evolution of superimposed bed forms on barchan and transverse dunes using the same model under different initial conditions we perform the linear stability analysis of a flat sand bed disturbed by a small sinusoidal perturbation comparing the most unstable wavelength in the model with the characteristic size of secondary bed forms in nature we determine the length and time scales of our cellular automaton model thus we establish a link between discrete and continuous approaches and open new perspectives for modeling and quantification of complex patterns in dune fields

late quaternary linear dune accumulation and chronostratigraphy of the southwestern kalahari implications for aeolian palaeoclimatic reconstructions and predictions of future dynamics abstract the linear dunes of the kalahari now largely inactive have long been identified as having potential palaeoenvironmental significance the application of optically stimulated luminescence osl dating to these dunes in the 1990s provided the first chronology of aeolian accumulation in this region though field methodologies and time consuming multiple aliquot laboratory protocols limited both the depth of sampling in dune bodies and the total number of samples dated in order to permit a more thorough investigation of the potential of these dunes to preserve long chronological records this intensive study presents 71 osl ages from the linear dunes of the southwestern kalahari at witpan south africa sampled with coring equipment at regular and frequent intervals down to bedrock the earliest sand accumulation recorded at witpan is at 104 ka and in spatially discrete locations other evidence of dune activity is recorded at 77 76 57 52 and 35 27 ka although an inherently discontinuous archive the linear dunes of the southwestern kalahari have the potential to record multiple phases of dune construction following the last glacial maximum there is near continuous evidence of dune building with a peak of accumulation recorded from 15 to 9 ka at five individual sites this latter period is generally recognised from other proxy evidence as being unusually arid in this region and such periods of dune activity are likely to be related to intensification of the continental anticyclone during the holocene accumulation has continued at most sites sampled albeit at a lesser intensity this may imply that these dunes are presently not far from thresholds of activation

a mid crustal linked ductile thrust zone breaghy head nw donegal ireland a discrete syn metamorphic ductile thrust zone in nw donegal is documented the imbricates link to a common detachment have well developed ramp flat geometries and duplicate a relatively simple well defined stratigraphic package similar in scale and geometry to imbricate zones in foreland thrust belts the ductile thrusts carry regionally significant displacement timing of imbrication with respect to development of major penecontem poraneous ductile thrusts and folds is consistent with a regional piggy back northwest propagating sequence of major syn metamorphic d2 age structures in nw donegal

sticky dunes in a wet desert formation stabilisation and modification of the australian desert dunefields independent dating evidence and dune morphology indicate great stability of the australian dunefields most dunefields have seen only minor superficial modification since they were formed up to 1 million years ago despite quite large changes in climate conditions this stability may be partly due to the relatively dense vegetation cover on australian dunes under the marginally arid climate but new studies supported by many older observations suggest that sticky dunes where sand grains are bonded or cemented may form under a broader range of wind climates than widely thought and have greater resistance to reworking new mapping of the australian continental dunefields from satellite imagery shows a previously unrecognised diversity of dune morphologies dune orientation continuity connectedness crest planform crest sharpness spacing and setting all show patterns of variation over the continent these are consistent with the overall low sand supply and variable wind climate that contribute to the dominance of longitudinal dunes but also with only superficial modification of the dunes after their initial formation the longevity of the dunes is likely also partly due to the stabilisation of dune sand by pedogenesis the bonding of sand by pedogenic calcium carbonate gypsum silica and translocated clays the extremely low mobility of the sand dunes has led to preservation of dunes of great age with stacked accretionary units and multiple palaeosols

temporal observations of a linear sand dune in the simpson desert central australia testing models for dune formation on planetary surfaces linear dunes are the most common dune form found on planetary surfaces yet questions remain about their formation temporal observations of a linear dune located in the simpson desert of central australia were made to monitor dune movement and to test competing hypotheses regarding linear dune formation our observations were collected on three separate occasions from 2006 to 2014 rebar stakes were placed in a gridded pattern so that multiple measurements of sand thickness gps surveys and photographs could be taken at the same locations over time we observed widespread reworking of sand on and around the dune crest with sand accumulation locally exceeding a meter between surveys overall the height of the dune crest increased by several centimeters we also observed fluctuations in the sand cover in the adjacent swales that often exceeded 2 3 cm between surveys yet we did not observe any appreciable changes in the position of the dune s downwind terminus weather data indicate that the effective sand transporting winds in the simpson are widely unimodal net sediment flux resultant drift direction is toward the north northwest locally at an oblique angle to dune orientation collectively our results suggest that the linear dune is actively maintained by vertical accretion the implications from our observations are that linear dunes on other planetary surfaces could form in wind regimes that are widely unimodal even where the resultant drift direction is locally oblique to dune orientation in particular such findings may provide support for global circulation models of titan

morphology development and sediment dynamics of elongating linear dunes on mars linear dunes occur on planetary surfaces including earth mars and titan yet their dynamics are poorly understood recent studies of terrestrial linear dunes suggest they migrate by elongation only in supply limited environments here we investigate elongating linear dunes in the hellespontus montes region of mars which are morphologically similar to terrestrial systems multi temporal high resolution orbital images show these linear dunes migrate by elongation only and that the fixed sediment source of the dunes probably restricts any lateral migration some linear dunes maintain their along length volume and elongate at rates comparable to adjacent barchans whereas those which decrease in volume show no elongation suggesting they are near steady state matching morphometric predictions limited sediment supply may restrict martian linear dunes to several kilometres significantly shorter than many terrestrial linear dunes our results demonstrate the close similarities in dune dynamics across the two planetary surfaces

the effect of wind speed and bed slope on sand transport this paper reports on a wind tunnel study of the effects of bed slope and wind speed on aeolian mass transport the use of a sloping wind tunnel has enabled estimation of the friction angle to be about 40 for saltating particles in the range 170 540 m a formula relating dimensionless mass transport to friction speed and bed slope is proposed and mass transport data for five uniform sand samples and one non uniform sand sample are shown to fit the equation well in particular the relationship reveals an overshoot in mass transport slightly above threshold collisions a feature also evident when previous experimental data is re examined as the number of mid air collisions between the saltating particles increases greatly with wind speed the overshoot may occur as a result of increasing energy losses resulting from the collisions finally it is demonstrated that data for saltating snow shows a similar overshoot in the dimensionless transport rate

mapping mud content and median grain size of north sea sediments a geostatistical approach abstract sediment grain size is well known for its influence on biogeophysical processes and hence grain size parameter maps important elements in an integrated ecological modelling strategy in this study a large database was compiled from legacy data on grain size parameters and distributions in north sea surface sediments the database was analysed by means of non linear regression to enable a consistent quantification of various grain size parameters in a second step multivariate geostatistics kriging were employed to predict the spatial distribution of percentage mud content and median grain size in the north sea with a resolution of 1 1 nautical miles the results show that incorporation of secondary information in the interpolation led to a physically more realistic representation of large scale patterns compared to deterministic approaches an evaluation of map confidence however suggests only minor differences in the quality obtained by different kriging techniques it appears that the data density and distribution are not an issue when it comes to performance instead insufficient metadata constrain the assessment and harmonisation of data sets and introduce uncertainty into the predictions

longitudinal dunes can move sideways longitudinal dunes occur in all major sand areas in the world their dominant mode of migration or extension is considered to be either prevailing or resultant wind parallel the dunes extending downwind via accretion of the terminal nose in the qaidam pendi in northwest china a series of active longitudinal dunes extend downwind at 5 10 m yr 1 internal sedimentary structures examined in the dunes however display beds dipping in one direction rather than two opposed directions as is expected analysis of aerial photographs confirms that these dunes migrate laterally up to three metres per year whilst maintaining a symmetrical longitudinal dune morphology

lateral migration of linear dunes in the strzelecki desert australia linear dunes in the strzelecki desert trend roughly south north sand transport which is toward the nne has caused the dunes to migrate eastward while they extend or migrate northward eastward lateral migration is evidenced by 1 asymmetrical shape of the dunes east facing slopes are several times as steep as west facing slopes 2 asymmetrical accumulation of loose recently transported sand relatively abundant on east facing slopes 3 asymmetrical outcropping of older semiconsolidated aeolian sand on the dune surface more abundant on west facing slopes and 4 east dipping foreset beds that underly the west facing flanks of some dunes dunes in the strzelecki desert are still active in the sense that sand is transported along and across many dune crests however the dunes are composed primarily of pleistocene strata indicating that the trend of the dunes was established before the holocene the obliquity of the dunes to the transport direction is not merely an aberration of the wind regime of the last few decades preferential accumulation of sand on east facing flanks indicates that the dunes migrated eastward several metres during the holocene moreover the west facing flanks of some dunes have experienced a minimum of tens of metres of erosion this asymmetric erosion and deposition were caused by dune obliquity and lateral migration that may have begun as early as the pleistocene dunes in the strzelecki desert and in the adjacent simpson desert display a variety of grossly different internal structures computer graphics experiments demonstrate that many of these differences in structure can be explained by different angles of climb of the dunes

why deposits of longitudinal dunes are rarely recognized in the geologic record dunes that are morphologically of linear type many of which are probably of longitudinal type in a morphodynamic sense are common in modern deserts but their deposits are rarely identified in aeolian sandstones one reason for non recognition of such dunes is that they can migrate laterally when they are not exactly parallel to the long term sand transport direction thereby depositing cross strata that have unimodal cross bed dip directions and consequently resemble deposits of transverse dunes dune parallel components of sand transport can be recognized in ancient aeolian sands by examining compound crossbedding formed by small dunes that migrated across the lee slopes of large dunes and documenting that the small dunes migrated with a component in a preferred along crest direction over the large dunes

monitoring surface change on a namib linear dune in tackling the apparently intractable problem of linear dune initiation and maintenance there has been a move away from large scale deductive models to smaller scale field studies of individual dunes this paper reports a study of surface change on a large complex linear dune in the namib desert southern africa the dune surface responds to a markedly seasonal wind regime in summer westerly winds erode sand from the west flank of the dune and deposit it on the easterly lee side of the dune crest in winter this pattern is reversed easterly winds erode sand from the east slope and deposit it on the west slope the crest therefore moves back and forth some 15 m each year returning at the end of a year s cycle to its position at the beginning the position of the base of the dune appears to remain fixed even though sand is moving throughout the dune system the dune does extend northward along some resultant of the westerly and easterly winds despite relatively high levels of activity especially at the dune crest there is no evidence of the breakdown of the linear dune form the conclusion must therefore be that linear dunes can be maintained in bimodal wind regimes and are not necessarily related to unidirectional parallel regimes as others have suggested

the geometric and kinematic development of the emma lake thrust stack grenville front southwestern labrador the emma lake thrust stack is a duplex structure consisting of basement cored fold nappes with thrust through overturned limbs developed in greenschist facies metamorphic rocks geometric relationships between fold nappes and thrust surfaces indicate both regular sequence and out of sequence thrusting occurred during development of the thrust stack microstructural analysis shows that regular sequence thrusts developed under ductile conditions whereas out of sequence thrusts developed in a brittle ductile field the structural geometry and metamorphic grade of the emma lake thrust stack indicate that it developed under ductile conditions at upper midcrustal levels the geometry of the emma lake thrust stack suggests that structural styles common to higher level fold and thrust belts may be traced into deeper levels of the crust

spatial patterns in gravel habitats and communities in the central and eastern english channel abstract the distribution of sediment type and benthic communities in the central and eastern english channel is shown to be polarised around a distinctive local hydrodynamic feature the seabed in the region includes an extensive area of gravel substrate which is both an important habitat for benthic marine fauna and a valuable source of material for the marine aggregate industry effective management of the area is predicated on an understanding of whether it represents a single homogeneous unit or several different units that may need to be managed in different ways the aim of this study was to provide information that would inform such management decisions spatial patterns in gravel habitats and communities were studied by investigating the physical environment through modelled and empirical data and the distribution of infauna and epifauna along an east west trending transect a common spatial pattern was observed in both physical and biological parameters but rather than indicating a simple longitudinal gradient there was a distinct polarisation around a central feature a bedload parting blp zone situated between the isle of wight and cotentin peninsula sediments and communities at the eastern and western ends of the transect were more similar to each other than to those in the middle the strong hydrodynamic regime in the blp area controls sediment distribution transporting finer material mainly sand away from the mid transect area the pattern in sand content of the substrate mirrors the magnitude of the potential bedload transport which is complex in this region due to the interplay between the m2 and m4 tidal constituents and produced a series of erosional and depositional zones the structure of benthic communities reflected the local substrate and hydrodynamic conditions with sponges observed among the stable substrates and stronger currents that characterised the mid transect area while infauna became more diverse towards the ends of the transect where substrates were more mobile we conclude that the area should not be considered as a homogeneous unit for management purposes despite its apparent uniformity on contemporary seabed sediment maps

11 11 dune morphology and dynamics this chapter discusses the key types of sand dunes and how they can be classified and assesses the major controls on their development the processes of dune dynamics are reviewed leading to a discussion of the concept of equilibrium in solitary dune form recent advances in modeling are reviewed that show how interactions between dunes may result in equilibrium at the dune field scale rather than at the single dune scale

a stratigraphic model to account for complexity in aeolian dune and interdune successions wet aeolian systems in which the water table or its capillary fringe are in contact with the accumulation surface such that moisture influences sedimentation are well known from modern aeolian systems and several ancient preserved successions are recognized from outcrop one common mechanism by which accumulation of wet aeolian system deposits occurs is via a progressive rise in the relative water table level that is coincident with ongoing dune and interdune migration the angle of dune climb being determined by the ratio between the rate of relative water table rise and the rate of downwind migration of the bedforms accumulations of wet aeolian system deposits tend to be characterized by units of climbing dune strata separated by units of damp or wet interdune strata for simple geometric configurations where the size of the dune and interdune units the rate of bedform migration and the rate of aggradation all remain constant over space and time the resulting accumulation has a simple architecture characterized by sets of uniform thickness inclined at a constant angle however the dynamic nature of most aeolian dune systems means that such simple configurations are unlikely in nature the complexity inherent in these systems is accounted for here by a numerical model in which key controlling parameters including dune and interdune wavelength and spacing migration rate and aggradation rate are allowed to vary systematically both spatially from a dune field centre to its margin and temporally in response to changes in sediment availability or water table level the range of synthetic stratigraphic architectures generated by the model accounts for all the best known examples of aeolian dune and interdune stratigraphic configurations documented from the stratigraphic record modelling results have enabled the erection of a scheme for the classification of dune system type whereby the many elaborate stratal architectures known to exist in nature can effectively be accounted for by only four parameters that are allowed to vary over space and time dune and interdune wavelength and spacing rate of bedform migration and rate of accumulation results have applied implications including the modelling of reservoir heterogeneity and the prediction of fluid flow pathways of hydrocarbons water co2 and contaminants in subsurface reservoirs and aquifers in which low permeability interdune units might act as baffles or barriers

environmental dynamics of a star dune abstract star dunes the largest aeolian bedforms in the sand seas of the world are usually distributed within specific geographical areas that have multi directional wind regimes however relatively few studies have focused on the environmental factors that impart such great volumes of sand to these dunes specifically verification of the developmental processes of star dunes through long term monitoring is scarce in this study by observing 3 d airflow fields and long term dune dynamics we demonstrate how topographic barriers which generate vertical airflow and local air circulation control the development of a star dune on mingsha mountain in dunhuang china results show that airflow stagnation and deflection caused by topography is one of the major mechanisms for the formation of star dunes in our study topographic barriers contribute to the development of intensive vertical airflow dominated by easterly winds this intensive vertical airflow is one of the main driving mechanisms of the upward growth of mega dunes vertical airflow is the strongest developed airflow reported in available data on aeolian geomorphology in addition star dunes are usually distributed in areas where the local air circulation is strong the results of long term dune dynamics verify that local air circulation which forms three wind directions with the regional wind regime contributes to the maintenance and development of star dunes our study indicates that complex mega dunes are products of topographic barriers which facilitate their recognition in aeolian geomorphology we introduce a new evolution pattern of star dunes under the influence of local environment and topographic barriers

organic facies of surface sediments in the north sea abstract surface sediments of the north sea were investigated by organic geochemical and sedimentological methods including organic carbon and nitrogen determination rock eval pyrolysis protokerogen microscopy gas chromatography of aliphatic hydrocarbons grain size fractionation and clay mineral analysis the total organic matter was found to be a mixture of mostly reworked or oxidized marine and terrestrial constituents with a generally strong influence of the latter throughout the platform locally improved hydrodynamic conditions yielded an incorporation of more freshly produced marine organic matter differences in water current velocities and turbulence also appeared to determine whether this supplement is incorporated into the fine or coarse fractions of the sediments a general constancy of the organic carbon contents within the individual size fractions was recognized independently of sediment type and age i e in recent and pre recent deposits this suggested that the latter have been effectively overprinted by the shelf processes operating at present or that these processes have been of comparable intensity since about the beginning of the holocene transgression the major factors controlling the spatial variations in the organic facies are the sediment grain size and the present day hydraulic regime

estimation of sediment grain sizes in a mesotidal area dithmarschen bight german north sea abstract this article presents a fast and suitable method to derive the spatial distribution of seabed grain sizes on tidal flats a successful validation was carried out in the dithmarschen bight a meso tidal area of the german north sea coast in particular the study is built on statistical correlations between observations of sediment sizes from 197 seabed samples and the numerical model output of flow wave variables under specific marine and atmospheric forcings the correlation between orbital velocities and mean grain sizes d50 exhibited the best results but it can be affected under low relative wave height ratios the methodology was applied for the construction of a high resolution map of grain sizes in the dithmarschen bight which resulted in absolute and relative errors of 16 m and 13 respectively the proposed relationship was also adopted to examine scenarios of field data scarcity

comparison of laser grain size analysis with pipette and sieve analysis a solution for the underestimation of the clay fraction classically the grain size of soil and sediment samples is determined by the sieve method for the coarse fractions and by the pipette method based on the stokes sedimentation rates for the fine fractions results from the two methods are compared with results from laser diffraction size analysis which is based on the forward scattering of monochromatic coherent light from a point of view of laboratory efficiency the laser sizing technique is far superior accuracy and reproducibility are shown by measurements on certified materials it appears that laser grain size measurements of certified materials correspond very well with the certificated measurements tests were also done on a set of randomly selected sediments of fluvial aeolian and lacustrine origin except for the 2 m clay fraction there is a coarsening of the mean diameter of one to two size classes 0 25 caused by the non sphericity of the particles the platy form of the clay particles induces considerable differences eight size classes between pipette and laser measurements the 2 m grain size defined by the pipette method corresponds with a grain size of 8 m defined by the laser particle sizer for the studied sediments using a higher grain size level for the clay fraction when laser analysis is applied enables workers in the geological and environmental field to compare classical pipette analysis with a laser sizing technique

mobility of sediment grain size distributions on a wave dominated continental shelf southeastern australia abstract seabed sediments comprise a full distribution of grain sizes previous models describing sediment mobility on the australian continental shelf have ignored the distribution of grain sizes presenting results for one size class only the mean grain size a simple model is presented which determines the probability of sediment entrainment which accounts for the full grain size distribution the model is applied to the wave dominated new south wales nsw continental shelf south eastern australia using observed grain size distribution data wave data from the port kembla waverider buoy are used to calculate sediment mobility in response to waves via two methods a simple empirical approach where maximum bed orbital velocity must exceed a critical bottom velocity and a more theoretically based approach where skin friction shear stresses must exceed a shields curve determined threshold for sediment entrainment results from the two methods compare to within 10 the effects of a variable sea are investigated by comparing sediment mobility in response to both significant and maximum waves maximum waves are found to mobilise medium fine sand fraction sediments on the mid shelf whereas significant waves mobilise medium fine sand fraction sediments on the inner shelf and only rarely the mid shelf maximum waves are capable of mobilising mud fraction size sediments if non cohesive beyond the shelf break combined wave wind generated current bed shear stresses were determined and indicate that waves dominate mobilisation on the inner and mid shelf but only wind driven currents are capable of mobilising sediments on the outer shelf during extreme wind events the distribution of bed shear stresses is consistent with the previously reported observed sediment distribution on the continental shelf determination of the sediment mobility of grain size distributions on the continental shelf is limited to available grain size distribution data consolidation of existing grain size datasets and newly collected data will allow for rapid extension of the method on a large scale

spatial prediction of seabed sediment texture classes by cokriging from a legacy database of point observations this paper illustrates the potential for statistical mapping of seabed sediment texture classes it reports the analysis of legacy data on the composition of seabed sediment samples from the uk continental shelf with respect to three particle size classes sand mud gravel after appropriate transformation for compositional variables the spatial variation of the sediment particle size classes was modelled geostatistically using robust variogram estimators to produce a validated linear model of coregionalization this was then used to predict the composition of seabed sediments at the nodes of a fine grid the predictions were back transformed to the original scales of measurement by a monte carlo integration over the prediction distribution on the transformed scale this approach allowed the probability to be computed for each class in a classification of seabed sediment texture at each node on the grid the probability of each class and derived information such as the class of maximum probability could therefore be mapped predictions were validated at a set of 2000 randomly sampled locations the class of maximum probability corresponded to the observed class with a frequency of 0 7 and the uncertainty of this prediction was shown to depend on the absolute probability of the class of maximum probability other tests showed that this geostatistical approach gives reliable predictions with meaningful uncertainty measures this provides a basis for rapid mapping of seabed sediment texture to classes with sound quantification of the uncertainty remapping to revised class definitions can also be done rapidly which will be of particular value in habitat mapping where the seabed geology is an important factor in biotope modelling

morphodynamic mechanisms for the formation of asymmetric barchans improvement of the bagnold and tsoar models asymmetric barchan dunes exhibit a marked asymmetry this morphology develops from the combined effects of two dominant winds with different directions and relative strengths and can be described based on bagnold s and tsoar s models in bagnold s model the limb nearest to the strongest wind extends and is sustained and enhanced by a gentler wind nearly parallel to the barchan tsoar s model expects that the limb farthest from the gentler wind extends in a manner similar to the evolution of seif dunes and that the limb closest to the gentler wind is eroded to know which model will emerge with variations of the wind we did some numerical simulations and found that there exists a critical angle between orientations of the two winds which decide the formation model of asymmetric barchans when the angle is greater than this critical value the effect of the bi directional winds agrees with tsoar s model when the angle is less than the critical value the evolution of the two horns contradicts tsoar s model and agrees with bagnold s model the critical value for this angle depends on the transport ratio of two winds

relationships between distributed and localized shear in the tectonic evolution of a caledonian fold and thrust zone northwest ireland broad zones of distributed shear operating through mid crustal regions of orogenic belts may incorporate narrow horizons of intense localized deformation culminating in discrete large magnitude displacements the relative importance and relationship between distributed and localized shear are influenced by a variety of factors including lithological variation pre existing structural anisotropy strain rate and migration of fluids rigorous structural analysis of lower amphibolite facies dalradian metasediments in northwestern ireland demonstrates that an early d 1 discrete ductile detachment was subsequently reactivated during distributed non coaxial d 2 deformation operating in a broad zone through the structural pile regional shear was directed towards the southeast and resulted in the generation and translation of kilometre scale isoclinal recumbent sheath folds which close and face towards the transport direction the d 1 detachment is clearly folded around the hinges of these major folds whilst on fold limbs it was reactivated and acted as a local decollement within the zone of distributed shear shear criteria along the detachment indicate a southeast directed translation of the major folds in sympathy with regional shear a broad zone of d 3 translation operating through the nappe pile resulted in coaxial refolding of large scale f 2 folds by the d 3 ballybofey nappe producing a complex fold interference pattern non coaxial d 3 deformation resulted in continued reactivation of local decollements together with the initiation of east southeast directed oblique thrusts and partial dismemberment of d 2 folds detailed structural investigation allows concepts of distributed and localized shear to be evaluated and models of crustal deformation to be assessed

particle size distribution analysis of sand sized particles by laser diffraction an experimental investigation of instrument sensitivity and the effects of particle shape laser diffraction is now widely used for particle size distribution analysis of sediments and soils the technique can be very precise and offers advantages of speed and cost over many other methods when used to analyse mixtures of sand silt and clay this study presents the results of an experimental investigation that examined the sensitivity of the beckman coulter ls230 instrument to mixtures of different grain populations and differences in particle shape the instrument was found to have high sensitivity to coarse particles in a finer matrix detection threshold 1 2 but much lower sensitivity to finer particles in a coarser mixture detection threshold 12 17 experiments using near spherical ballotini showed that laser analysis provides very similar values to dry sieving for the mean median and mode but for a range of natural sand samples values for the mean median and mode were offset by 8 21 with an average of ca 15 compared with sieving analysis using a beckman coulter rapidvue instrument which provides both size and shape information provided support for the hypothesis that the differences between laser analysis and sieving are partly attributable to the effects of particle shape however an additional factor is the way in which the laser software interprets the optical diffraction data the software predicts a high degree of log normality in the size distribution such that highly skewed truncated or bimodal samples are poorly represented experiments using sieved fractions of ballotini indicated that even with near perfectly spherical particles the particle size distribution predicted by the laser software includes a relatively large percentage of particles outside the sieve class limits

grain size and depositional environment as predictors of permeability in coastal marine sands more than half of the surface sediments covering the continental shelves are sandy which may permit substantial sub seafloor pore water advection knowledge of sediment permeability is required for quantifying advection and associated solute transport but studies of marine sediments typically report grain size analyses rather than permeability here data from 23 studies were examined to determine the range in permeabilities reported for sublittoral marine sands and to assess the utility of permeability grain size relationships in this setting in the resulting database the permeability of small w30 cm undisturbed cores collected from the sea floor all fell between 2 10 12 and 4 10 10 m 2 a range where advective transport induced by wave and current action should be pervasive the range in grain size was very similar for near shore 10 m water depth but the permeability of the continental shelf samples was consistently lower for the same median grain size empirical permeability grain size relationships generated a poor fit r 2 0 35 for the aggregate data but separate relationships for near shore and continental shelf samples were significantly better r 2 0 66 and 0 77 respectively permeability grain size relationships thus may be useful for sublittoral sands but a larger database needs to be accumulated before reliable fit parameters and variability can be predicted thus it is recommended that permeability be routinely determined when characterizing sedimentological properties of marine sand deposits concurrent determinations of sediment bulk density and porosity may further improve estimates of permeability 2008 published by elsevier ltd

tectonic and kinematic evolution within mid crustal orogenic root zones a case study from the caledonides of northwestern ireland the slieve league peninsula of northwest ireland lies on the western limb of a major orogenic strike swing in which regional foliation trends have deviated from the northeast southwest trends typical of much of scotland to west east orientations across strike coastal exposures on the western tip of the peninsula through neoproterozoic dalradian metasediments enable a detailed examination and analysis of the structural evolution of a caledonian orogenic root zone which has been previously correlated with the loch awe syncline of southwest scotland minor structural development may be evaluated in terms of regional strain profiles and overprinting relationships over much of the area a composite steep northeast southwest trending s2 s3 foliation containing a gently southwest plunging quartz mineral elongation lineation is the dominant fabric at outcrop and is associated with mp2 almandine amphibolite facies metamorphism f2 folds are isoclinal with curvilinear hinges and similar geometry they typically plunge steeply towards the southwest and display variable dextral or north directed vergence whilst minor f3 fold hinges plunge moderately towards the southwest and typically verge sinistrally towards the south major composite d1 d3 tectonic slides are developed in the argyll group structural and stratigraphic relationships indicate that d1 induced large scale reversals in younging across tectonic slides resulting in reversals in subsequent f2 and f3 facing patterns tectonic sliding is associated with an intensification of strain demonstrated by increasingly intrafolial and curvilinear folding together with extensional crenulations sheared quartz pods and metre scale asymmetric boudinage of metadolerites all of which indicate dextral d2 and sinistral d3 shear after unfolding subsequent folds f4 this corresponds to top to the north d2 and top to the south d3 translations d4 results in regionally northwest verging structures with minor crenulations and the s4 cleavage transecting fold hinges in an anticlockwise sense suggesting a dextral component of deformation the detailed kinematic data indicate that the overall geometry of this western deep level arm of the root zone is not a product of the classic mushrooming fountain of nappes model but rather major interference between consistent northerly directed d2 thrusting and a later phase of southeast directed d3 retrocharriage back folding which intensifies towards the south

classifying seabed sediment type using simulated tidal induced bed shear stress abstract an ability to estimate the large scale spatial variability of seabed sediment type in the absence of extensive observational data is valuable for many applications in some physical e g morphodynamic models knowledge of seabed sediment type is important for inputting spatially varying bed roughness and in biological studies an ability to estimate the distribution of seabed sediment benefits habitat mapping e g scallop dredging although shelf sea sediment motion is complex driven by a combination of tidal currents waves and wind driven currents in many tidally energetic seas such as the irish sea long term seabed sediment transport is dominated by tidal currents we compare observations of seabed sediment grain size from 242 irish sea seabed samples with simulated tidal induced bed shear stress from a three dimensional tidal model roms to quantitatively define the relationship between observed grain size and simulated bed shear stress with focus on the median grain size of well sorted seabed sediment samples we present predictive maps of the distribution of seabed sediment classes in the irish sea ranging from mud to gravel when compared with the distribution of well sorted sediment classifications mud sand and gravel from the british geological survey digital seabed sediment map of irish sea sediments digsbs250 this grain size tidal current proxy gstcp correctly estimates the observed seabed sediment classification in over 73 of the area

extensional geometries as a result of regional scale thrusting tectonic slides of the dunlewy nw donegal area ireland abstract the synmetamorphic ductile dislocations known in the british caledonian literature as tectonic slides pose a classical structural problem that is despite being associated with synchronous contractional folds and cleavages the low angle dislocations have the effect in many celebrated cases of juxtaposing younger over older rocks a geometry normally associated with extensional rather than contractional deformation recent models have attempted to demonstrate that this is the result of thrust reactivation of original sedimentary extensional growth faults the appin group dalradian metasediments of the complex and small dunlewy area of nw donegal ireland contain the following geometric elements a an early strike swing related stratigraphic facies change b a major inter deformational dolerite sheet c major regional recumbent folds and slides d major structures related to the 400 ma sinistral main donegal granite shear zone this solution to the structural geometry reveals that the early mid crustal 11 km depth d 2 ardsbeg dunlewy slide is a thrust to the northwest its hangingwall contains rocks two thirds of which are younger than the rocks of the footwall together with major recumbent folds coeval with the underlying thrust which face downwards into the thrust in the direction of transport rather than thrust reactivation of an original extensional growth fault we find that both stratigraphic and structural constraints are satisfied by a double thrusting model with fault bend folding onto an upper ramp of an earlier formed but penecontemporaneous and kinematically linked major fold pair this solution to the geology also allows us to recognize that the regional pre granite structure of the dalradian of nw donegal is a series of major d 2 synmetamorphic thrust bounded nappes possibly involving up to 250 km of northwesterly overthrusting

bedform alignment in directionally varying flows many kinds of sediment bedforms are presumed to trend either normal or parallel to the direction of sediment transport for this reason the trend of bedforms observed by remote sensing or by field observations is commonly used as an indicator of the direction of sediment transport such presumptions regarding bedform trend were tested experimentally in bidirectional flows by rotating a sand covered board in steady winds transverse oblique and longitudinal bedforms were created by changing only two parameters the angle between the two winds and the proportions of sand transported in the two directions regardless of whether the experimental bedforms were transverse oblique or longitudinal as defined by the bedform trend relative to the resultant transport direction they all had trends that yielded the maximum gross transport across the bedforms the fact that many of the experimental bedforms were neither transverse nor parallel to the resultant transport direction suggests that transport directions cannot be accurately determined by presuming such alignment

internal structure and surface geometry of longitudinal seif dunes abstract a mutual relationship exists between the morphology of longitudinal seif dunes and their dynamics and internal structure the basic mechanism of longitudinal dune movement results from the deflection of oblique wind flow into a longitudinal flow on the lee flank the magnitude of the longitudinal flow depends on the angle of the wind incidence with the crest line deposition occurs when this angle of incidence increases through meandering of the longitudinal dune crest line after heavy rain exposed the internal structure of the dune two kinds of depositional unit were found the main unit forms in the meandering area on both sides of the dune the other secondary unit is a narrow belt on both sides and parallel to the crest line the two units have different strikes because the laminae deposited on the dune are of the three primary stratification types climbing translatent strata sand flow cross strata and grainfall laminae the range of dips is wide 10 to 30 these dips accord with topographic profiles across the dune a model of the internal structure of a longitudinal dune has been developed from the field data the model shows the erosional and depositional areas along this linear dune

two modes for dune orientation earth s sand seas dune fields experience winds that blow with different strengths and from different directions in line with the seasons in response dune fields show a rich variety of shapes from crescentic barchans to star and linear dunes these dunes commonly exhibit complex and compound patterns with a range of length scales and various orientations which up to now have remained difficult to relate to wind cycles here we develop a model for dune orientation that explains the coexistence of bedforms with different alignments in multidirectional wind regimes this model derives from subaqueous experiments which show that a single bidirectional flow regime can lead to two different dune orientations depending on sediment availability i e the erodibility of the bed sediment availability selects the overriding mechanism for the formation of dunes increasing in height from the destabilization of a sand bed with no restriction in sediment availability or elongating in a finger on a non erodible surface from a localized sand source these mechanisms drive the dune orientation therefore dune alignment maximizes dune orthogonality to sand fluxes in the bed instability mode while dunes are aligned with the mean sand transport direction in the fingering mode applied to earth s deserts the model quantitatively predicts the orientation of rectilinear dunes and their superimposed patterns this field study suggests that many linear dunes on earth elongate from sources and are simply aligned with the mean sand transport direction

origin and morphology of barchan and linear clay dunes in the shuhongtu basin alashan plateau china abstract clay dunes are characterized by a relatively high content of silt and clay and are typically developed on the lee side margin of closed basins in arid and semi arid areas although they are widely distributed aeolian landforms on earth little is known about their development and distribution in asia our field investigations have identified for the first time the occurrence of clay dunes in the alashan plateau north china the clay dunes occur in three isolated lowland areas of the suhongtu sht basin and their morphologies barchans hooked barchans asymmetrical barchans and linear or seif dunes differ substantially from those of the well studied clay dunes in australia africa and north america these differences could be mainly attributed to the asymmetrical bimodal wind regime prolonged drought and sediment physical properties which indicate that dunes of any type can be formed in sand silt or clay based on the known evolution of the sht paleo lake basin and regional climate change we infer that the landscape of modern clay dunes in the desert of the alashan plateau does not predate mis 5 and probably formed during the late holocene analyses of grain size distributions and the contents of trace and rare earth elements combined with geomorphological characteristics indicate that the source sediments of the clay dunes are mainly local cretaceous tertiary and quaternary red fluvial lacustrine mudstone and sediments while the ultimate source is the weathered and denuded products of granitoids of the east altay mountains comparison of the morphology and alignment of the clay dunes with the prevailing wind pattern tends to support tsoar s 1984 model of the transition in dune shape from barchans to seif or linear dunes during 2003 2013 the seif or linear dunes of the sht basin migrated laterally and elongated northeastward at rates of about 2 4 m yr and 16 20 m yr respectively

how human activities affect the fine sediment distribution in the dutch coastal zone seabed abstract the fine sediment distribution in the seabed is an important indicator for the ecological functioning of shallow coastal seas in this paper we investigate the processes and conditions that determine the fine sediment distribution in the dutch coastal zone surficial seabed while also assessing the response of the system to human interventions an extensive sediment dataset collected in the dutch coastal zone from 2006 to 2014 is presented these data are used to map the distribution of fines in the seabed of the dcz at unique spatiotemporal scales for the entire dutch coastal zone the distribution of fines generally agrees well with previous studies the recent extension of the port of rotterdam the maasvlakte 2 reclamation was found to locally change the distribution of fines in the sand mining pit and directly south of the reclamation fines percentages in the seabed increased by more than 10 we developed a conceptual framework to analyse the distribution of fines and how it is affected by human interventions three components are distinguished within this framework 1 sources of fines 2 transport pathways and 3 accumulation potential these components are determined both qualitatively and quantitatively based on high resolution bathymetric and hydrodynamic model data the distinction between the three components makes it possible to unravel the contributions of different human interventions to the changes in the fines distribution in the case of maasvlakte 2 the local increase of fines percentage in the seabed could thus be attributed to a temporary additional source of fines and enhanced accumulation potential the high spatiotemporal resolution of the new sediment dataset proved crucial to enable development and testing of the framework to evaluate the impact of large engineering works on the spatial distribution of fines

storm controlled oblique dunes of the oregon coast the large mean height 25 m spacing 300 m relatively straight crested dunes of the central oregon coast migrate an average of 3 8 m yr toward an azimuth of 26 the dunes are transverse to the strong south southwesterly winter storm winds that are responsible for their basic form orientation and migration the dry moderate north northwesterly summer winds modify the dune form but not the dune trend comparison of the sand transport calculated from wind data and the transport measured from dune migration indicates that the actual transport by the wet southerly winds is only one third of the amount calculated assuming dry conditions the resultant vector mean transport rate as recalculated by comparison of the measured and initially calculated rates is 34 m 3 m yr toward an azimuth of 45 the dunes are thus oblique by our definition of an oblique dune angle between dune trend and resultant transport direction between 15 and 75 the internal structures of the dunes confirm northward migration during wet conditions evidence for deposition during wet conditions includes slipface deposits deformed mostly by sliding and various structures formed by the adhesion of sand grains to wet surfaces most summer deposits are not preserved but those on the basal apron the gentle north slope at the base of the winter slipface have a high preservation potential a depositional model based on dune climbing predicts that the preserved record of oblique dunes formed by an obtuse bimodal wind regime would consist of tabular sets of crossbeds in which the dip angles increase upward from the base of each set

fine particle retention and deposition in regions of cyclonic tidal current rotation abstract benthic sediments in continental shelf seas control a variety of biogeochemical processes yet their composition especially that of fine sediment remains difficult to predict mechanisms for mud or fine sediment deposition and retention are not fully understood using sediment data and a hydrodynamic model of the northwest european shelf seas a relationship is shown to exist between fine benthic sediment composition and regions of cyclonic tidal current rotation the reduced thickness of cyclonic tidal benthic boundary layers compared with the anticyclonic case promotes deposition of fine sediment and trapping of resuspended material adding the effects of the benthic boundary layer thickness as influenced by ellipticity or not sheds some light on the limitations of approaches only focusing on bed shear stress and sediment pathways to predict the location of mud deposits a tidal boundary layer predictor that includes ellipticity alongside tidal current magnitude and depth was shown to spatially agree with maps of mud deposits

long term measurements of aeolian transport directional variations over a zibar surface in the northern kumtagh sand sea abstract zibars are aeolian bedforms created by armoring of the surface by coarse grains the existence and movement of these grains strongly influence zibar morphodynamics although aeolian sediment transport has been investigated for arid surface processes with a range of grain sizes the measurement durations were short making it difficult to establish the relationship between microscopic aeolian transport processes and macroscopic geomorphological processes in the present study we conducted field observations of aeolian sediment transport above a zibar in northern china s kumtagh sand sea with a segmented sand trap during six measurement periods throughout 2014 the sampler collected 314 76 kg of sediments within 1 m above the ground the directional variation indicated that about 52 5 of the sediments came from the north northeast and northwest the southeast provided the least sediments the strongest aeolian transport occurred between august and october the weakest was in winter the aeolian sediment flux decreased with height and 54 of particles were transported within 0 1 m above the ground the transport profiles fit an exponential decay function the flux s directional variation resulted from the combined effects of sediment availability and wind regime so the calculated net transport differed from the drift potential the seasonal and directional variations of aeolian transport significantly affect the morphodynamic process of zibars the trend and the internal sedimentary structure of zibars were closely related to the aeolian transport process

multiple origins of linear dunes on earth and titan the formation of dunes is controlled by the direction of the prevailing winds and the characteristics of the sediments linear dunes in the qaidam basin china are shown to form from cohesive sediments a model that could be applicable to dunes on titan

eolian dunes computer simulations and attractor interpretation a simple computer simulation algorithm for the transport of sand by wind produces forms resembling barchan crescentic ridge linear and star natural dune classes sand is moved as slabs composed of many grains that are picked up at random transported in a specified direction and deposited 1 with a probability that depends on the local presence or absence of sand or 2 in shadow zones in the lee of dunes the simulated dune fields are interpreted as complex systems with sand dune classes being dynamical attractors of these systems the evolution of dunes once formed becomes decoupled from the details of eolian sand transport

sediment flux from the morphodynamics of elongating linear dunes although dunes are very common bedforms in terrestrial sand seas the description of linear dune growth either by extension or lateral accretion is still hindered by our limited understanding of the underlying mechanisms therefore sand flux estimates from remote imagery rely essentially on the migration speed of barchan dunes but not on the dynamics of linear dunes here we use 50 yr of high resolution aerial and satellite imagery of the tenere desert niger the world s largest source of mineral aerosols to demonstrate that linear dunes can elongate in the direction of the resultant sand flux with no lateral migration as they elongate from topographic obstacles in a zone of low sediment availability with multimodal winds these elongating lee dunes are ideal to isolate and quantify linear dune growth only by extension using similar conditions in a numerical model we show how deposition downstream of low hills may result in nucleation and development of bedforms from elongation we derive the local sand flux parallel to the linear dune crests this study shows that the morphodynamics of linear dunes under complex wind regimes can also be used for assessing sediment flux and wind conditions comparably to the more established method of using sand flux estimates perpendicular to the barchan dune crests in zones of unidirectional wind

structures and fabrics in a crustal scale shear zone betic cordillera se spain abstract a broad zone of dominantly ductile high strain deformation lies beneath the aguilon nappe in the sierra alhamilla southern spain it forms part of a crustal scale movement zone traceable through much of the betic cordillera which separates the higher betic nappes from the underlying nevado filabride complex the zone is characterized in outcrop by a distinctive platy foliation and a strong nne trending stretching lineation microstructural characteristics include quartz ribbons mica fish augen of feldspar and other minerals in a matrix of dynamically recrystallized quartz and extensional crenulation cleavages narrow bands of ultramylonite and cataclasite occur within and on the margins of the movement zone deformation occurred under lower greenschist facies conditions and was accompanied by retrogression of earlier higher grade mineral assemblages structures in the movement zone developed in a temporal sequence beginning with isoclinal folding and transposition of older foliations this was followed by the formation of extensional crenulation cleavages and the progressive localization of strain into the ultramylonite bands mylonitic foliation in these bands is deformed by syn mylonite folds restricted to the bands all these structures were then deformed by s to se vergent small scale folds restricted to the movement zone as a whole cataclasis associated with alteration is localized along the ultramylonite bands and indicates a transition to late stage brittle deformation the lower boundary of the movement zone is gradational strain decreases recrystallized grainsize and the degree of recrystallization of quartz increases and pressure solution becomes the dominant deformation mechanism in mica schist asymmetric quartz fabrics in the movement zone indicate a nne sense of shear but variations in the degree of asymmetry suggest that flow was partitioned with the ultramylonite bands taking up much of the shear strain and the intervening rocks deforming more slowly and with a lower degree of non coaxiality diffuse fabrics in the fine grained ultramylonite bands may indicate a switch to a grainsize sensitive deformation mechanism and an overall downward increase in the opening angle of crossed girdle fabrics may reflect increased water activity at depth

emergence of oblique dunes in a landscape scale experiment the dynamics of dune evolution under bimodal wind regimes are poorly understood owing to a lack of long term wind records and the limitations of most experimental set ups a 4 year landscape scale experiment in the tengger desert mongolia demonstrates that the orientation of oblique dune crests is controlled by the wind regime