formation and stability of transverse and longitudinal sand dunes the shape of dunes depends on the history of wind regimes and sand availability in deserts exposed to winds from two different directions but with comparable magnitude dunes are found to be linear ridges which are either perpendicular or parallel to the mean wind direction depending on the angle between the two wind directions these dunes respectively observed for small and large angles between winds are called transverse and longitudinal dunes in both cases their large width hundreds of meters and evolution time scale years strongly limit the investigation of their dynamics and thus our understanding of such structures here we show that under water similar structures can be obtained but at much smaller space and time scales performing controlled experiments together with numerical simulations we highlight the physical mechanisms at play in the formation and long term evolution of these structures we show in particular that while longitudinal dunes are stable and extend in time transverse dunes are unstable they evolve into wavy ridges and eventually break into barchans if the sand supply is too low this fundamental difference is understood through the study of single sand piles and bars exposed to two winds in the case of a large angle between winds a sand pile grows a finger pointing in the average wind direction and transforms into a longitudinal dune such an elongation does not occur for a small angle where a sand pile evolves into a barchan these results explain the morphological differences between straight and long longitudinal dunes and sinuous transverse dunes while giving keys to infer the wind history or pattern state of development from the observation of dune shapes in the field

eolian topography as a control on deposition incorporating lessons from modern dune seas permian cedar mesa sandstone se utah u s a abstract the dominant current model for the accumulation of eolian strata in the rock record emphasizes translation and climb of bedforms and the preservation of continuous beds studies of modern dune fields however show that when large dunes form they create a topographically irregular surface later episodes of eolian activity commonly fill the low lying areas between the older dunes resulting in beds that are laterally discontinuous strata in part of the permian cedar mesa sandstone in southeastern utah require this model in this new model inherited eolian topography influences subsequent deposition detailed gps mapping of an 8 km2 area was used to trace eolian strata through a 20 25 m thick unit in the cedar mesa sandstone the unit is bounded above and below by subparallel erosion surfaces that have up to 5 m of relief within the unit large transverse draa or megadune deposits are spaced 1 km apart locally over 0 5 km wide areas the draa deposits fill the entire 25 m thickness of the unit only to be completely replaced by interdraa deposits a few hundred meters away the draa surfaces were eroded and exposed prior to the filling of the intervening lows with alternations of barchanoid dune deposits sand sheets and ponds which formed in the topographic lows the cedar mesa sandstone illustrates aggradation wherein the important factor is the preserved eolian topography the draas formed an undulating stable surface from which subsequent deposition filled the interdraa low large eolian dunes that influenced subsequent deposition are also observed in several pleistocene to recent dune seas this mechanism offers an alternative to the existing models that require continuous migration of large dunes

secondary flow deflection in the lee of transverse dunes with implications for dune morphodynamics and migration measurements of lee side airflow response from an extensive array of meteorological instruments combined with smoke and flow streamer visualization is used to examine the development and morphodynamic significance of the lee side separation vortex over closely spaced transverse dune ridges a differential deflection mechanism is presented that explains the three dimensional pattern of lee side airflow structure for a variety of incident flow angles these flow patterns produce reversed along dune and deflected surface flow vectors in the lee that are inferred to result in net lateral diversion of sand transport over one dune wavelength for incident angles as small as 10 from crest transverse i e 80 from the crest line this lateral displacement increases markedly with incident flow angle when expressed as the absolute value of the total deflection in degrees reversed and multi directional flow occurs for incident angles between 90 and 50 these results document the three dimensional nature of flow and sand transport over transverse dunes and provide empirical evidence for an oblique migration model copyright 2013 john wiley sons ltd

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

high altitude aeolian research on the tibetan plateau aeolian processes and their role in desertification have been studied extensively at low elevations but have been rarely studied at high elevations in areas such as the tibetan plateau where aeolian processes were active in the geologic past and remain active today in this review we summarize research that improves our understanding of aeolian processes on the tibetan plateau including the distribution characteristics and provenance of aeolian sediments the history of aeolian activity aeolian geomorphology and wind driven land degradation contemporary aeolian processes primarily occur in dry basins in wide river valleys on lakeshores on mountain slopes and on gravel pavements sediment characteristics suggest a local origin and provide interesting contrasts with those of china s loess plateau the history of aeolian activity and its paleoclimatic implications reconstructed based on aeolian archives is short mostly since the late glacial and shows wide regional differences aeolian geomorphology is simple and suggests short formation time wind driven land degradation is less severe than previously thought driven by different factors in different areas and exhibited complex interactions with freeze thaw processes aeolian research has been conducted within the general framework of aeolian science but addresses issues specific to the tibetan plateau that arise due to the low air temperature low air density and the presence of a cryosphere we propose six priorities for future research aeolian physics the effect of freeze thaw cycles comparisons with other areas regional differences effects of wind driven land degradation and integrated observation and monitoring

interdune deposits of the namib sand sea abstract the namib sand sea of southwestern africa is dominated by large north south trending linear dunes interdune areas however comprise about 50 of the area of the sand sea and may consist of exposures of the pre dune surface or sediments deposited between the dunes four major types of interdune deposits are identified and described 1 1 coarse poorly sorted aeolian sands 2 2 localized calcareous lacustrine deposits 3 3 silty deposits of ephemeral rivers that have flooded into the sand sea margins 4 4 sabkhas and salt marshes near the coast the preservation potential of these sediments is poor except where they are buried and protected by dunes or stabilised by high groundwater levels in coastal areas

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

the morphology of permian palaeodunes a reinterpretation of the bridgnorth sandstone around bridgnorth england in the light of modern dune studies abstract an examination of the permian bridgnorth sandstone formation around bridgnorth shropshire has indicated that it probably accumulated in a combination of transverse and barchanoid draas with superimposed oblique crescentic and linear dunes under the influence of a fluctuating wind regime three major recurrent facies associations were identifiable isolated transverse draa with superimposed oblique crescentic and linear dunes barchanoid draa with superimposed oblique crescentic and linear dunes and bimodal sand sheets with isolated dome dunes the overall long term wind direction was probably from the east and appears to have controlled draa orientation the orientations of the superimposed dunes however were probably related to medium term wind fluctuations from a more northerly quarter possibly modified by the draa bedforms short term wind reversals may have reworked slipfaces as shown by the presence of internal scour surfaces within dune and draa foresets

temporal and spatial variability in dune reactivation across the nebraska sand hills usa the nebraska sand hills is a stabilized dune field on the great plains of north america although it is well known that this dune field like several others on the great plains last experienced widespread activity during the medieval climatic anomaly mca ad 900 1300 spatial variation in the timing and nature of drought development is poorly constrained to elucidate spatial trends in dune reactivation samples potentially representing mca activity across the sand hills were collected and dated using optically stimulated luminescence osl ages from the older part of the mca were obtained from eolian sediments in the northwestern sand hills while ages from later in the episode were obtained to the southeast suggesting a geographic trend in the timing of revegetation of the dunes near the end of the drought revegetation likely occurred to the northwest initially as a result of renewed moisture availability from a rising water table in the interdunes which serve as refugia for vegetation during times of drought vegetation then gradually spread to the southeastern sand hills an additional spatial trend in ages is apparent in the chronology of linear dune mobilization across the sand hills linear dunes in the northwest are superimposed on megadunes and originated during the last reactivation while linear dunes in the southeast are built around older cores of dunes and formed during several reactivations our geochronology reveals three episodes of eolian transport including the mca in the formation of linear dunes in the southeast

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

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

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

application of spatial cross correlation to detection of migration of submarine sand dunes knowledge of migration rates of bedforms provides an indirect indication of thebehavior of tidally averaged bottom currents enables optimization of hydrographic surveyfrequency and may enable calculation of bedload transport rate to measure bedformmigration rate we test the use of spatial correlation as a measurement method whichquantifies and locates a region of maximum similarity between two spatial variables forthe latter we use consecutive eight bit images of spatial gradient derived frombathymetric digital terrain models carrying out the correlation over this representation ofthe shape of the seabed rather than the bathymetric surface the digital terrain modelswere compiled from six repeat multibeam surveys of a headland associated bank nearsaint john new brunswick with a roughly 30 day interval vectors are drawn depictingthe movement of a sand dune at time t

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

aeolian sands in continental red beds of the middle buntsandstein lower triassic at the western margin of the german basin abstract aeolian sands occur widespread in continental red beds of the middle buntsandstein lower triassic at the western margin of the german basin middle europe in the eifel area cross bedded sands were mainly deposited by grainfall on lee slopes of barchanoid type dunes in unimodal wind regime in both low energy and high energy environments occasionally sets are internally truncated by reactivation surfaces crest height of solitary set dunes varies in the range of some meters locally sequences of cosets both of tabular planar and wedge planar type capped by flat or gently windward dipping topset beds represent larger dune complexes of about 10 20 m in height and at least 80 100 m in lateral extent horizontal laminated deposits originated as sand sheets in interdune areas the dune sands were deposited by trade winds of the northern hemisphere in low palaeolatitude predominantly by southeasterly and southwesterly trade winds in summer when the intertropical convergence zone was shifted to the north aeolian sands built up an extensive dune belt in the eifel intersected by braided to anastomosing rivers fluviatile incursions or heavy ephemeral rainfall led to aquatic redeposition of aeolian sediments and origin of shallow lakes in interdune depressions distribution of aeolian and fluviatile sediments in karlstal schichten is different in the southern western and northern eifel middle buntsandstein sequence in the southern eifel reflects an evolution of the fluviatile depositional environment also incorporating the occurrence and distribution of aeolian sands evolution is characterized by increasing spacing and sinuosity of channels weakening supply of coarse detritus from the source areas and decreasing braiding of the river systems aeolian sands occur in the final phase of this evolution which led from local alluvial fans via cobbly and pebbly braided rivers to sandy braided streams and finally to an intertonguing of aeolian dunes and braided to anastomosing rivers matching with the climax of regional diversification of depositional milieu the aeolian sand sea in the eifel covered an area of nearly 2500 km2 at the time of its largest extension the dune belt ended in the northern eifel the alternating sequence passed into an entirely alluvial succession which graded northwards by continuously diminishing grain size into the central playa lacustrine to marine sediment series in northwest germany palaeogeography sedimentology and genesis of the middle buntsandstein at the western margin of the mid european basin and of rotliegendes in northwest germany appear to be partially similar as is revealed by certain characteristics of the palaeogeographical reconstructions concurrence of middle buntsandstein palaeogeographical models for both eifel and the whole german basin most clearly delineates the key position of the eifel for interpretation of the buntsandstein sedimentary environment and the palaeogeography in middle europe

lateral migration thermoluminescence chronology and colour variation of longitudinal dunes near birdsville in the simpson desert central australia the longitudinal dunes of the simpson desert in the vicinity of birdsville have been reworked largely during the holocene from dunes deposited up to 80000 years ago or earlier the widespread asymmetry of these roughly northward trending dunes with steeper eastern faces and more gentle western faces supports wind rose data showing sand transporting winds from the southwest obliquely intersecting the dunes while this suggests a change in the wind pattern since the dune field was oriented it does not indicate that the dunes are necessarily shifting leeward eastward as a consequence it is hypothesized that the direction of migration is controlled by the extent to which the dunes are vegetated relatively well vegetated dunes can accrete sand on their gentle stoss slopes and erode on their lee slopes causing them to shift westward and hence obliquely into the wind a condition that probably prevails in wetter regions and during episodes of relatively humid climate in contrast in very dry areas or during arid phases sand can move unimpeded up a sparsely vegetated stoss face and over the crest to form an avalanche or slip face on the lee side thereby causing the dunes to shift eastward despite evidence that longitudinal dune crests can shift laterally to some extent the dunes in the western part of the simpson desert have not migrated either westward or eastward more than 100m or so from their pleistocene cores aeolian transport and partial or complete removal of iron cutans from around quartz grains results in dunes of widely varying colour yet of similar age

superposition of aeolian dust ripple patterns as a result of changing wind directions the effect of a change of wind direction on the orientation and some other morphometric characteristics of aeolian dust ripples is investigated ripple formation is simulated in a wind tunnel on surfaces that are already characterized by a previously established rippling the effect of a wind rotation of 45 degrees 90 degrees and 180 degrees is examined it is found that wind rotations of 45 and 180 degrees will lead towards a ripple alignment perpendicular to the last air flow a wind rotation of 90 degrees however leads towards a ripple alignment perpendicular to the resultant wind direction the faster the wind blows the more this effect is expressed no distinct relationship exists between the asymmetry of a dust ripple and a dust ripple field and the direction of the wind blowing as the ripples formed as the orientation and asymmetry of aeolian dust ripples do not necessarily reflect the air flow direction during which the ripples formed great care should be taken when reconstructing wind directions from such aeolian ripples

the utility of desert sand dunes as quaternary chronostratigraphic archives evidence from the northeast rub al khali abstract the distributions of optically stimulated luminescence osl ages from dunes are affected by palaeoenvironmental changes complex dune dynamics and sampling strategy extracting the relative importance of these factors when interpreting discontinuous osl chronologies from sand dunes has proven difficult and is particularly hindered in contexts where the internal sedimentary structures of dunes are not visible in this study samples for osl dating were taken from three major dune exposures in the rub al khali united arab emirates each showing clear internal structure with the aim of addressing these problems specific objectives were to assess how ages of individual sedimentary units represent dune accumulation as a whole and how the dune record in turn reflects known past environmental changes final ages were calculated using the oxcal software package by incorporating the known relative stratigraphy through the application of bayesian methods the results show that stratigraphy alone is not sufficient at these sites to guide osl sampling that is chrono and lithostratigraphic boundaries do not necessarily coincide where chronological hiatuses are present internal sediment stratigraphy can be a useful tool in identifying potential problems of under sampling the full dune record the implications of these findings for reconstructing quaternary climates from dune chronologies are considered

sedimentological characteristics and morphology of the aeolian sand dunes in the eastern part of the uae a case study from ar rub al khali abstract the umm az zimul al wijan area is located in the southeastern part of the uae and covers about 1600 km 2 it is bounded to the east by the oman mountains and to the south and west by the extensive dune area of ar rub al khali field investigation revealed the common occurrence of parallel to sub parallel sand ridges with a general east west orientation also present but less frequent are barchan barchanoid and star dunes interdune areas are classified into four types that include dolomite mixed ancient sabkha and ophiolitic rock fragments mixed sand sheet and ophiolitic rock fragments and recent sabkha the textural characteristics of the sand forming the dunes are unimodal with a modal class in fine sand size moderately to very well sorted positively skewed and mesokurtic quartz grains are mainly subrounded to subangular the similarity in roundness properties in different dune types in the eastern province of the uae seems not to be influenced by dune form based on the heavy mineral suites and the angularity of quartz grains it is believed that the sand sediments are mostly derived from the ultrabasic rocks forming the ophiolitic sequence of the oman mountains acidic rocks of iran and from the coastal area

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

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

two types of longitudinal dune fields and possible mechanisms for their development longitudinal dune fields characterized by nearly uniform interdune spacing are distinguished from longitudinal dune fields characterized by fairly variable interdune spacing and high frequencies of dune coalescence the empirical and theoretical evidence indicating that the former may be due to helical air currents aligned with the dunes is reviewed hypotheses arguing that the latter may arise indirectly from horizontal pressure gradients or bidirectional wind regimes are discussed evenly spaced linear sand banks aligned with tidal currents may be shown mathematically to result from energy optimalization within two dimensional sand transporting flow regimes and a similar simple or non rotational flow model is considered for the problem of desert longitudinal dunes an initial complex or rotational flow analysis is undertaken to discern the likely significance of roll vortices in desert sediment transport an evolutionary timescale is estimated for the formation of desert longitudinal dune fields a simple analysis is performed for the effect of regional sand mass change on longitudinal dune field ordering recommendations are made for future empirical and theoretical research

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

morphology internal structure and mechanics of small longitudinal seif dunes in an aeolian horizon of the proterozoic dhandraul quartzite india the excellently preserved metre scale linear bedforms in an aeolian horizon of the proterozoic dhandraul quartzite india show oppositely dipping strata arranged in a zigzag pattern the strata are dominantly of translatent type deposited by along crest migrating ripples preserved on the flanks of dunes the bedforms thus may be interpreted in a morphodynamic sense as longitudinal seif dunes in order to determine the regional palaeoflow pattern the migration directions of ripples preserved at the top of sheet sandstones that are associated with the dune cross strata and internally show subhorizontal translatent strata were measured a directionally varying flow with a mean direction nearly parallel to the mean axial trend of the dunes is indicated the kinematics of the dunes were thus largely the result of alternate operation of two oblique flow components each of which was deflected at a dune crest into an along crest flow on the downwind flank of the dune the deflected flow formed along crest migrating ripples which in turn deposited climbing ripple strata alternate deposition on the two opposite flanks resulted in near vertical accretion of the dunes as is indicated by the zigzag pattern of stratal arrangement

the development of large aeolian bedforms abstract large aeolian bedforms complex and compound varieties of dunes or draas dominate over wide areas of many sand seas and are well represented in the rock record analyses of allometric relationships between dune height and spacing in several sand seas suggest that draas develop by vertical growth of existing dunes in the namib and other sand seas draa tend to occur in areas of low total and net sand transport rates the distribution of such bedforms in a sand sea reflects long term patterns of deposition which are related to regional scale changes in wind regimes and sand transport rates superimposed bedforms are almost ubiquitous on draa and develop only after a minimum size of draa has been reached formation of superimposed dunes reflects spatial and temporal variations in secondary flow patterns developed on the draa measurements of winds and depositional patterns on complex linear dunes in the namib sand sea suggest that an important mechanism for their growth is the migration of superimposed dunes from areas of high to low wind velocity on draa flanks leading to the deposition of sediment by bedform climbing

grain size variations on a longitudinal dune and a barchan dune abstract the grain size characteristics of the sand upon two dunes a 40 m high longitudinal dune in the central namib desert and a 6 0 m high barchan in the jafurah sand sea of saudi arabia vary with position on the dunes on the longitudinal dune median grain size decreases sorting improves and the grain size distributions are less skewed and more normalized toward the crest though sand at the windward toe is distinct elsewhere on the dune the changes in grain size characteristics are gradual an abrupt change in grain size and sorting near the crest as described by bagnold 1941 pp 226 229 is not well represented on this dune coarse grains remain as a lag on concave slope units and small particles are winnowed from the sand on the steepest windward slopes near the crest avalanching down slipfaces at the crest acts only as a supplementary grading mechanism on the barchan dune median grain size also decreases near the crest but sorting becomes poorer though the grain size distributions are more symmetric and more normalized the dune profile is a gaussian curve with a broad convex zone at the apex upon which topset beds had accreted prior to sampling grain size increases and sorting improves down the dune s slipface however this grading mechanism does not influence sand on the whole dune because variations in wind regime bring about different modes of dune accretion on both dunes height and morphology appear to influence significantly the grain size characteristics

late pleistocene aeolian and fluvial interactions in the development of the nizzana dune field negev desert israel the interdunal areas in the nizzana linear sand dune field contain both sandy and silty sediments a series of trenches was excavated across the interdunal corridor exposing stacked sequences of silty and sandy units which are locally restricted to palaeodepressions the silty units contain fining upward sequences and are interpreted as overbank deposits from the nahal nizzana thermoluminescence dating and identification of buried palaeosols indicates that the silt and clay layers were deposited over a period of several thousands of years in the late pleistocene the sands between the silt layers have been fluvially reworked and are not primary aeolian deposits the stacked sequences of fluvial deposits indicate that the palaeodepressions persisted in the landscape for a significant time attesting to long term stability of the interdunal areas it also suggests that the linear dunes themselves have not moved laterally during this time despite climatic changes and devegetation since the end of the late pleistocene the nahal nizzana has downcut and overbank deposition no longer occurs within the interdunal corridors the playa deposits today are positive relief features indicating that topographic inversion has occurred and that the interdunal areas are geomorphically active

internal structure of a trough blowout determined from migrated ground penetrating radar profiles ground penetrating radar gpr was used to investigate the relationship between the geomorphological development of a large aeolian trough blowout and the stratigraphy and internal sedimentary structure of its associated deposits although analogous many of the data processing techniques routinely applied in seismic reflection are very rarely applied in gpr studies in this study a simple migration program was used that significantly enhanced the quality of gpr images from a large trough blowout at raven meols on the sefton coast northwest england these improvements aided subsequent data interpretation which was achieved through application of the principles of radar stratigraphy gpr shows the pre blowout dunes to have a complex internal structure that suggests they were formed in the presence of at least a partial vegetation cover subsequent to stabilization of these dunes a thin soil developed this dune soil forms an important radar sequence boundary and delineates a complex topography beneath the depositional lobe of the blowout the internal structure of the depositional lobe of the blowout does not conform to a model of simple radial foreset deposition as derived from contemporary process studies reported in the literature instead the pattern of deposition has been extensively modified by the antecedent dune topography and by varying spatial and temporal exposure to important sand transporting winds that is partly controlled by interactions between the regional wind pattern and local dune morphology trough blowout deposits in coastal aeolian sedimentary sequences are likely to be recognized by the presence of laterally continuous packets of relatively high angle cross strata which often display a spatially variable radial dip pattern that is only very poorly or partially developed in addition a soil or other surface representing a significant hiatus in dune deposition is likely to underlie the blowout deposits the topography of which will show a clear relationship to the dip and orientation of the overlying cross strata

evaluating the use of dune sediments as a proxy for palaeo aridity a southern african case study abstract the dominance of dryland environments in the southern hemisphere makes the study of these regions of critical importance for the development of regional hemispheric and global models of environmental change unfortunately the wetting and drying cycles associated with semi hyperarid climates are not conducive to the preservation of traditional organic proxy data sources the last decade however has seen the development of a number of alternative archives including the application of luminescence dating techniques to dunes and other aeolian deposits in continental situations the existence of relict dune fields has long been thought to be evidence of drier conditions during the pleistocene and direct ages from these features have been interpreted almost exclusively as indicating phases of aridity however an increasing number of ages from a broader range of environments are calling into question the assumption that aeolian activity can be simply equated with aridity presented here is a comparison of dune ages that have been obtained from across southern africa with a range of proxies from both terrestrial and marine records taken as a whole three primary phases of activity can be identified at 60 40 35 20 and 17 4 ka the frequent discordance with other terrestrial records indicating coeval increases in humidity and the close correlation of these phases with wind strength proxies suggest that aridity is unlikely to be the sole or even primary forcing mechanism for aeolian activity in the region and the palaeoclimatic significance of these sedimentary archives needs to be reassessed

geomorphology of desert sand dunes a review of recent progress through the 1980s and 1990s studies of the geomorphology of desert sand dunes were dominated by field studies of wind flow and sand flow over individual dunes alongside these there were some attempts numerically to model dune development as well as some wind tunnel studies that investigated wind flow over dunes as developments with equipment allowed field measurements became more sophisticated however by the mid 1990s it was clear that even these more complex measurements were still unable to explain the mechanisms by which sand is entrained and transported most importantly the attempt to measure the stresses imposed by the wind on the sand surface proved impossible and the use of shear or friction velocity as a surrogate for shear stress also failed to deliver at the same time it has become apparent that turbulent structures in the flow may be as or more important in explaining sand flux in a development paralleled in fluvial geomorphology aeolian geomorphologists have attempted to measure and model turbulent structures over dunes progress has recently been made through the use of more complex numerical models based on computational fluid dynamics cfd some of the modelling work has also suggested that notions of dune equilibrium form may not be particularly helpful this range of recent developments has not meant that field studies are now redundant for linear dunes careful observations of individual dunes have provided important data about how the dunes develop but in this particular field some progress has been made through ground penetrating radar images of the internal structure of the dunes the paradigm for studies of desert dune geomorphology for several decades has been that good quality empirical data about wind flow and sand flux will enable us to understand how dunes are created and maintain their form at least some of the difficulty in the past arose from the plethora of undirected data generated by largely inductive field studies more recently attention has shifted although not completely to modelling approaches and very considerable progress has been made in developing models of dune development it is clear however that the models will continue to require accurate field observations in order for us to be able to develop a clear understanding of desert sand dune geomorphology

understanding linear dune chronologies insights from a simple accumulation model abstract dunefields are today often found in regions where environmental conditions are no longer conducive to widespread aeolian transport and deposition and are thus seen as potential archives of palaeoenvironmental information some dune types are more suitable for this purpose than others and linear dunes used here synonymously with longitudinal dunes have been most frequently employed they are commonly found in dryland and dryland marginal regions and they are apparently less migratory than some other dune forms thereby offering the potential for longer environmental histories most recent studies have employed optical osl dating surveys which directly date the emplacement of dune sands to attempt to produce a representative history of aeolian accumulation at a dunefield scale yet such studies have not always given due consideration to the geomorphology of linear dune accumulation although linear dunes do not rework their sand as readily as say barchans dunes it is clear that sand deposited during an aeolian event is likely to have been removed from elsewhere on the dunefield s surface for this reason all dune archives must be assumed to be discontinuous over long timescales the effect of sediment redistribution on dune preservation and thus palaeoenvironmental interpretation is difficult to observe and poorly understood we present a simplified probabilistic model to replicate deposition erosion of sand at a linear dune crest thus simulating the type of profile sampled in many field studies in order to produce a local accumulation history the modelled sand flux incorporates various feedbacks and the simulations are run with varying boundary conditions in which the simulated surface is assumed to be in equilibrium over the multi millennial timescales of the model s duration periodic instabilities in the landscape are generally well recorded by the modelled dune accumulation history particularly when the events involve the system as a whole briefly functioning out of an equilibrium state i e the probabilities of deposition and reworking are not equal however under forcing conditions where the probabilities are unchanged throughout the duration of the model run the results can show apparent structure within the data when sample sizes are low although at large sample sizes the preservation potential decays with burial age according to a power law the model confirms the importance of rigorous sampling strategies for such studies but also suggests that if such precautions are taken the reworking inherent to linear dune aeolian systems does not preclude their use as a quaternary palaeoenvironmental archive of environmental change

exploration for oil accumulations in entrada sandstone san juan basin new mexico recent exploration activity in the san juan basin of northwestern new mexico has resulted in the discovery of new oil fields in the entrada sandstone of jurassic age the major trapping element is provided by topographic relief in excess of 100 ft 30 m on top of the entrada preliminary analyses indicate that the topographic relief was created by preserved eolian sand dunes which were formed in a topographic basin which then became the sight of a large lake in which limestones and anhydrites of the todilto formation were deposited over the entrada sands the organic rich limestones of the todilto provide the most likely source for the oil found in the underlying entrada analysis of stratigraphy oil shows source rock potential and porosity distribution led to the selection of an initial exploration area located along the southeastern flank of the san juan basin seismic model studies confirmed by an experimental seismic program indicated that the topographic relief on top of the entrada could be mapped seismically an extensive seismic and drilling program has resulted in the discovery of six new oil pools in addition to the topographic relief on top of the entrada other factors which control the oil accumulations include local structural conditions hydrodynamics source rock and oil migration history and porosity permeability relations in the entrada the knowledge gained from this exploration program should aid in future exploration for entrada oil fields in the san juan basin and encourage exploration for similar stratigraphic traps in other basins

new ph sensor for hydrothermal fluids the ph values of subsurface hydrothermal fluids remain controversial and largely unknown because of the difficulties of measuring them in situ this paper presents a new method for evaluating the ph of hydrothermal fluids from analyses of halogen concentrations in hydrous minerals this method is general and can be used to study water rock interactions in a variety of geologic environments an example of the method9s application to the salton sea geothermal systems california is given

preservation of interdraa and plinth deposits by the lateral migration of large linear draas lower permian yellow sands northeast england abstract the lower permian yellow sands are preserved as eight relatively wide draa ridges linear compound dunes separated by interdraa corridors at the base of the marl slate zechstein flat bedded interdraa and draa plinth deposits are a significant constituent of the deposits exposed in the draa ridges these sediments are composed of relatively coarse grained wind ripple strata and form up to 9 m thick tabular sand bodies interdraa deposits are generally the most coarse grained and are horizontally to sub horizontally stratified they are gradationally overlain by low angle draa plinth deposits with numerous low angle diastems the contact between draa plinth and overlying trough cross bedded draa centre deposits is frequently gradational but can also be characterized by deep aeolian scours each draa ridge contains two interdraa draa plinth draa centre units which record an early phase of lateral migration of the draa field followed by a second phase of vertical accretion

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

large wind shift on the great plains during the medieval warm period spring summer winds from the south move moist air from the gulf of mexico to the great plains rainfall in the growing season sustains prairie grasses that keep large dunes in the nebraska sand hills immobile longitudinal dunes built during the medieval warm period 800 to 1000 years before the present record the last major period of sand mobility these dunes are oriented nw se and are composed of cross strata with bipolar dip directions the trend and structure of the dunes record a drought that was initiated and sustained by a historically unprecedented shift of spring summer atmospheric circulation over the plains moist southerly flow was replaced by dry southwesterly flow

transverse and linear dunes in an upper pleistocene marine sequence corinth basin greece a range of large scale dunes of oolitic calcarenite composition are exposed in the corinth basin of central greece these transverse dunes and a very large linear dune 15 m high lie within an upper pleistocene transgressive marine sequence tidal flow accelerated by constriction through a narrow fault bounded seaway is interpreted to have generated the current velocities necessary to produce the dunes marine facies in the upper pleistocene sequence include beach to offshore conglomerates and sandstones with wave modified sedimentary structures and herringbone cross stratification an offshore facies association comprises variably bioturbated siltstones and sandstones with a varied marine fauna that includes thermophile species such as scleractinian corals and strombus bubonius oolitic sandstone facies also occur oolitic sands were apparently produced in shoal environments subject to tidal and wave action and transported by dominant southerly currents over the southern part of the basin oolites accumulated in a linear dune 2 7 km long and 15 20 m high and in three dimensional transverse dunes up to 10 m high having a variety of compound and simple internal geometries the isolated wsw ene trending linear form exhibits angle of repose sedimentary dips up to 35 of avalanche sets on its se flank and sets typically with dips of 15 20 to the nw internal high angle discontinuities are developed in the se dipping lee face it is proposed that a dominant north to south flow crossed over the crest obliquely resulting in both net erosional and depositional processes on the lee flank a subordinate tidal current may have locally and or periodically crossed the dune crest in a westwards direction a string of transverse dunes which were located adjacent to a fault marine terrace scarp is interpreted to have originally coalesced to form the linear dune the distribution of transverse and linear dunes together with the palaeogeographical reconstruction suggest that a marine connection periodically existed across the corinth isthmus during the late pleistocene due to a combination of active faulting and glacio eustatic highstands of sea level

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

methodology for reconstructing wind direction wind speed and duration of wind events from aeolian cross strata 1 a methodology for reconstructing wind direction speed and event duration from aeolian dune cross strata was developed from analysis of crescentic dunes at white sands new mexico during wind events dune lee faces were surveyed lee face deposits mapped deposition rates measured grain size sampled by stratification type and winds characterized from meteorological and field data the spatial distribution of lee face stratification styles is a function of the incidence angle formed between the wind and the brinkline with secondary controls by wind speed and dune sinuosity and height sets of wind ripple strata form at incidence angles of 25 40 grainfall grainflow foresets over wind ripple bottomsets at 40 70 and grainflow grainfall foresets at 70 90 erosional reactivation surfaces form at incidence angles up to 15 bypass surfaces up to 25 the total sediment load is fractionated within lee face stratification types wind speed can be reconstructed from relationships between grain size transport mode shear velocity and grain settling velocity where the full range of grain transport modes occurs and grain size is limited by shear stress the shear velocity and grain size range in each transport mode can be estimated by assuming the coarse fraction in grainflow strata traveled in creep and the coarse fraction in grainfall traveled in saltation the minimum duration of a wind event can be estimated using measures of shear velocity dune height and dune forward migration method limitations arise with source area control on grain size extremes in wind events and severe truncation of sets of cross strata

dynamic processes of a simple linear dune a study in the taklimakan sand sea china samples of dune sands surveys of the morphology and field measurements of wind velocity and direction of a simple linear dune in taklimakan sand sea show that the airflow and sand flux vary with the change of wind direction on the dune surface decrease of the airflow stress on the lee flank does not result in much decrease of the sand flux because of the low threshold shear velocities and the airflow conditions there are no significant relations between the sand flux on the lee flank and the angle of incidence of the airflow the low threshold shear velocities and the maintenance of the sand flux at the lee flank are the main mechanisms keeping the linear shape of the dunes measurements of the sand flux shows that it reaches a maximum on the crest of the dune the grain size of the transported sands has some differences compared to that of the dune surface the sands transported are finer than that on the dune surface but better sorted under the influence of the medium to low wind activity the field experiment results exhibit that it is possible for the dunes to be shaped as linear dunes during the processes of accumulation and elongation

desert aeolian origin of late cenozoic regolith in arid and semi arid southwestern australia abstract using a sedimentary geology approach the late cenozoic regolith of southwestern australia is interpreted as basement saprock and saprolite unconformably overlain by sequences of continental sedimentary deposits the sedimentary deposits which overlie saprock and saprolite are allochthonous variously altered and mainly partly to wholly desert aeolian sediments they include aeolian and fluvial sandy clayrock facies e g deposits which resemble the pallid and mottled zones of weathered precambrian bedrock of the traditional laterite profile altered aeolian sand and sandy dust facies e g hardpan lower parts of duplex soils red sandy silt claystone and calcrete valley fills altered sandy duststones e g bauxite laterite and ferricrete of the traditional laterite profile and bioturbated aeolian sand sheets e g sandplain soils lateritic sandplains of the traditional laterite profile and upper parts of duplex soils the allochthonous and desert aeolian views proposed here differ fundamentally from the traditional residual colluvial and erosional views consequently there is a need for both traditional and alternative views to be critically appraised from the perspective of sedimentary geology

a method for semi automated objective quantification of linear bedforms from multi scale digital elevation models the increasing availability of large detailed digital representations of the earth s surface demands the application of objective and quantitative analyses given recent advances in the understanding of the mechanisms of formation of linear bedform features from a range of environments objective measurement of their wavelength orientation crest and trough positions height and asymmetry is highly desirable these parameters are also of use when determining observation based parameters for use in many applications such as numerical modelling surface classification and sediment transport pathway analysis here we 1 adapt and extend extant techniques to provide a suite of semi automatic tools which calculate crest orientation wavelength height asymmetry direction and asymmetry ratios of bedforms and then 2 undertake sensitivity tests on synthetic data increasingly complex seabeds and a very large scale 39 000 km2 aeolian dune system the automated results arecompared with traditional manually derived measurements at each stage this new approach successfully analyses different types of topographic data from aeolian and marine environments from a range of sources with tens of millions of data points being processed in a semi automated and objective manner within minutes rather than hours or days the results from these analyses show there is significant variability in all measurable parameters in what might otherwise be considered uniform bedform fields for example the dunes of the rub al khali on the arabian peninsula are shown to exhibit deviations in dimensions from global trends morphological and dune asymmetry analysis of the rub al khali suggests parts of the sand sea may be adjusting to a changed wind regime from that during their formation 100 to 10 ka bp

a unifying model for planform straightness of ripples and dunes in air and water abstract geologists physicists and mathematicians have studied ripples and dunes for more than a century but despite considerable effort no general model has been proposed to explain perhaps the most fundamental property of their morphology why are some bedforms straight continuous parallel and uniform in planform geometry i e two dimensional whereas others are irregular three dimensional here we argue that physical coupling along the crest of a bedform is required to produce straight crests and that along crest flow and sand transport provide effective physical mechanisms for that coupling ripples and dunes with the straightest and most continuous crests include longitudinal and oblique dunes in unidirectional flows wave ripples dunes in reversing flows wind ripples and ripples migrating along a slope at first glance these bedforms appear quite different ripples and dunes air and water transverse oblique and longitudinal orientations relative to the net sand transport direction but they all have one property in common a process that increases the amount of along crest sand transport that lengthens and straightens their crests relative to the across crest transport that makes them migrate and take the more typical and more three dimensional planform geometry in unidirectional flows that produce straight bedforms along crest transport of sand is caused by along crest flow non transverse bedform orientation gravitational transport along an inclined crest or ballistic splash in air bedforms in reversing flows tend to be straighter than their unidirectional counterparts because reverse transport across the bedform crest reduces the net across crest transport that causes the more typical irregular geometry relative to the along crest transport that smoothes and straightens planform geometry

dune deformation in a multi directional wind regime white sands dune field new mexico as with most dune fields the white sands dune field in new mexico forms in a wind regime that is not unimodal in this study crescentic dune shape change deformation with migration at white sands was explored in a time series of five lidar derived digital elevation models dems and compared to a record of wind direction and speed during the same period for the study period of june 2007 to june 2010 244 sand transporting wind events occurred and define a dominant wind mode from the sw and lesser modes from the nnw and sse based upon difference maps and tracing of dune brinklines overall dune behavior consists of crest normal migration to the ne but also along crest migration of dune sinuosity and stoss superimposed dunes to the se the sw winds are transverse to dune orientations and cause most forward migration the nnw winds cause along crest migration of dune sinuosity and stoss bedforms as well as se migration of ne trending dune terminations the sse winds cause ephemeral dune deformation especially crestal slipface reversals the dunes deform with migration because of differences in dune segment size and differences in the lee face deposition rate as a function of the incidence angle between the wind direction and the local brinkline orientation each wind event deforms dune shape this new shape then serves as the boundary condition for the next wind event shared incidence angle control on dune deformation and lee face stratification types allows for an idealized model for white sands dunes copyright 2015 john wiley sons ltd

mesoproterozoic erg and sand sheet system architecture and controlling factors galho do miguel formation se brazil abstract aeolian systems of the precambrian time were established in different palaeoclimatic and palaeogeographic conditions than the present different morphologies of dunes draas as linear oblique and star forms are widespread in the recent aeolian systems however the ancient aeolian deposits are rarely interpreted as these types of bedforms and usually referred to as transverse dunes the nature and complexity of bedform morphology in precambrian aeolian systems as well as the factors that allow the construction accumulation and preservation of these environments are still not well defined extensive exposures of galho do miguel formation in southeastern brazil represent a mesoproterozoic erg succession and exhibit a distinct facies architecture which contributed to the understanding of these deposits three architectural elements were identified i tangential cross stratified sandstone with superimposed trough cross bedded sandstone ii planar laminated sandstone and iii extensive flat bedded sandstone dominated by wrinkly laminae these elements are organized into three succession groups representative of simple transverse dune oblique compound draa and sand sheet systems the simple to compound dune draa system corresponds to the most central parts of the erg associated with a higher sediment availability and lower water table level conversely the sand sheet system occurs under low sediment availability and higher water table level in the marginal portions of the erg precambrian climatic oscillations may have changed the aeolian system morphology allowing the alternation of simple and complex aeolian dunes resulting in high frequency changes in sand sheet system and the remobilization of previously accumulated sediments over large geographic areas

distinctions and uses of stratification types in the interpretation of eolian sand abstract eolian and subaqueous cross strata cannot be distinguished reliably by many commonly cited criteria they can however generally be distinguished by the characteristics of their component types of stratification which represent processes of sorting and transport of the grain population on dunes eolian dune stratification types consist of grainflow cross strata grainfall laminae and wind ripple generated climbing translatent strata some of these types especially translatent strata have characteristics unique to the eolian realm these same stratification types are found to compose some ancient cross strata and their occurrence confirms the eolian interpretations of pans of the entrada jurassic navajo triassic jurassic and galesville cambrian formations a well as revealing emergent islands in the curtis formation jurassic previously considered to be totally marine in origin stratification types show a characteristic distribution on modern eolian dunes and differ in their relative abundances and structure on dunes of differing size and kind these same relations allow some estimates of the type shape and original height of ancient dune deposits as well as influencing the occurrences of surface features such as tracks and tipple forms the geological record of stratification types and other dune features is greatly affected however by the extent of the post depositional truncation of dunes