laboratory simulation of clast abrasion brewer paul lewin j 2002 laboratory simulation of clast abrasion earth surface processes and landforms 27 pp 145 164 rae2008

on the analysis of longitudinal stress in glaciers analyse des contraintes et des deformations dans un glacier lorsque les contraintes longitudinales deviatrices sont comparables aux contraintes de cisaillement a la base on etudie aussi le cas ou la composante du poids vers l aval n est pas equilibree par la contrainte de cisaillement basale on obtient les expressions de la contrainte de cisaillement a la base en fonction de l epaisseur de la glace de la pente du gradient de deformation en surface et des proprietes de la glace

sand dunes as a major proximal dust source for late pleistocene loess in the negev desert israel grain size analyses of three hilltop primary eolian loess sequences in the negev desert southern israel show a bimodal grain size distribution at 50 60 m and 3 8 m using analyses of mineralogy and osl ages we demonstrate that the coarse mode is composed mostly of quartz grains and its relative magnitude increases regionally with time suggesting an enhancement of a time transgressive proximal dust source compared to a distal saharan fine grain dust the only proximal dust source for large amount of coarse silt quartz grains is the sands that advanced into sinai and the negev concurrently with the loess accretion during the late pleistocene as a result of the exposure of the mediterranean shelf we therefore propose that the coarse silt quartz grains were formed through eolian abrasion within the margins of an advancing sand sea this relationship between desert sand seas as a source for proximal coarse dust and desert margin loess deposits can be applicable to other worldwide deserts such as northern africa china and australia

aeolian abrasion and modes of fine particle production from natural red dune sands an experimental study a series of experiments was conducted to determine the potential for aeolian abrasion of natural dune sands to produce fine particles 125 m by 1 the release of resident fines 2 spalling chipping and breakage of particles and 3 the removal of grain surface coatings parent samples were obtained from the surfaces of four active continental dunes and abraded using a glass test tube chamber for up to 120h the fine particles produced by this abrasion process were trapped at varying time intervals and subject to detailed particle size analyses using a coulter multisizer the abrasion of untreated parent samples produced fine particles in one of two main size classes 50 m but when the parent sample was sieved to exclude particles 250 m relatively more material in the range 10 50 m was produced for unsieved parent samples the size range associated with the dominant mode varied according to the length of the abrasion time the coarsest mode 63 m was dominant during the first 16 h of abrasion then became less significant and is thought to be associated with the release of resident fines into suspension the finest mode 10 m was absent or very weak during the first 16h of abrasion then became more significant and in some instances dominated the distribution as abrasion continued removal of grain surface coatings is the main source of fine material 10 m and this may be a significant source of fine material in areas where sands are dominated by subrounded and rounded particles by comparison with previous studies of aeolian particle abrasion these natural dune sands produced very low quantities of fine material by weight but their spatial extent makes them potentially a significant source of dust sized particles at the global scale

controls on the large scale spatial variations of dune field properties in the barchanoid portion of white sands dune field new mexico previous studies have shown that sediment fluxes and dune sizes are a maximum near the upwind margin of the white sands dune field and decrease to first order with increasing distance downwind these patterns have alternatively been attributed to a shear stress overshoot associated with a roughness transition localized at the upwind margin and to the influence of long wavelength topography on the hydrology and hence erodibility of dune field sediments i point out an issue that compromises the shear stress overshoot model and further test the hypothesis that long wavelength topographic variations acting in concert with feedbacks among aerodynamic granulometric and geomorphic variables control dune field properties at white sands building upon the existing literature i document that the mean and variability of grain sizes sand dryness aerodynamic roughness lengths bed shear stresses sediment fluxes and ripple and dune heights all achieve local maxima at the crests of the two most prominent scarps in the dune field one coincident with the upwind margin and the other located 6 7 km downwind computational fluid dynamics cfd modeling predicts that bed shear stresses erosion rates and the supply of relatively coarse poorly sorted sediments are localized at the two scarps due to flow line convergence hydrology and the spatially distributed adjustment of the boundary layer to variations in dune size as a result the crests of the scarps have larger ripples due to the granulometric control of ripple size larger grain sizes and or larger ripples lead to larger dunes and hence larger values of bed shear stress in a positive feedback

sorting out abrasion in a gypsum dune field 1 grain size distributions in eolian settings are the result of both sorting and abrasion of grains by saltation the two are tightly coupled because mobility of particles determines abrasion rate while abrasion affects the mobility of particles by changing their mass and shape few field studies have examined this quantitatively we measured grain size and shape over a 9 km transect downwind of a line sediment source at white sands national monument a gypsum dune field the sediment source is composed of rodlike elongate coarse particles whose shapes appear to reflect the crystalline structure of gypsum dispersion in grain size decreases rapidly from the source coarse particles gradually become less elongate while an enrichment of smaller more elongate grains is observed along the transect transport calculations confirm that white sands is a threshold sand sea in which 1 the predominant particle diameter reflects grains transported in saltation under the dune forming wind velocity and 2 smaller elongate grains move in suspension under this dominant wind size selective transport explains first order trends in grain size however abrasion changes the shape of saltating grains and produces elongate smaller grains that are spallation and breaking products of larger particles both shape and size changes saturate 5 6 km downwind of the source as large particles become more equant abrasion rates slow down because protruding regions have been removed such asymptotic behavior of shape and abrasion rate has been observed in theory and experiment and is likely a generic result of the abrasion process in any environment

shallow hypersaline lakes as paleoclimate archives a case study from the laguna salada malaga province southern spain abstract although numerous studies concerning the holocene climate of the southern iberian peninsula were accomplished within the last few decades the climate history of this region is still poorly understood various studies deal with a combination of proxies which are neither easy to compare nor is their connection easy to explain e g due to spatial patterns and time transgression within this study the suitability of the lacustrine sediments from the laguna salada andalucia region southern spain as a paleoclimate archive is investigated the lake sediments were evaluated using a multi proxy approach including sedimentological mineralogical geochemical and biological analyses the sediments reflect the evolution of the lake from pre medieval times onwards and characeae as well as ostracod analyses give an indication of paleosalinity moreover does the geochemical composition provide profound information concerning changes of elemental and mineralogical composition nevertheless a robust high resolution chronology could not be achieved owed to the scarcity of material available for radiocarbon dating and contamination problems furthermore poor preservation of pollen restricted the reconstruction of vegetation history which could have complemented important information concerning climatic changes and human activity

factors affecting the nature and rate of dust production from natural dune sands this paper evaluates the influence of natural sand particle characteristics on the amount and particle size distributions of dust produced by aeolian abrasion it contrasts with previous studies of aeolian abrasion by conducting experiments using i whole sand samples as opposed to selected size fractions ii natural mature dune sands rather than artificial or freshly crushed material and iii weathered sands that have acquired a superficial clay coating instead of grains with clean surfaces whilst previous research has found clear positive relationships between particle size sorting roundness and the amount of dust produced by aeolian abrasion the relationships determined in this study show some variation according to the geomorphological context from which the original samples were obtained the most important factor affecting the amount and particle size characteristics of the dust produced was the presence of a clay coating on the grain surface that is removed by the abrasion process the dust produced by this mechanism had a modal size of 2 5 m and material 10 m comprised up to 90 of the particles produced

making loess sized quartz silt data from laboratory simulations and implications for sediment transport pathways and the formation of desert loess deposits associated with the sahara abstract results from a series of laboratory simulations designed to investigate quartz silt generation by a number of geomorphic mechanisms are reviewed these findings are then used to discuss potential sediment pathways involved in the formation of desert loess deposits and a pathway is presented to show a proposed sequence of events involved in the formation of the loess deposits associated with the sahara the results indicate that both fluvial and aeolian systems can be very effective at producing quartz silt it is suggested that fluvial systems within hot desert environments may play a central role in both the production and distribution of loessic silt first the highly energetic but periodic fluvial regimes associated with arid and semi arid environments may be responsible for considerable quantities of particle comminution second the intermittent nature of these systems will help maintain geomorphologically active surfaces from which silt sized material can be easily deflated as well as fluvial comminution a range of weathering mechanisms and aeolian abrasion may also be responsible for the generation of silts in desert loess consequently silt sources and mechanisms of silt generation are likely to be much more diverse for desert loess than those associated with glacial loess

mid pleistocene hominin occupation at elandsfontein western cape south africa abstract the current understanding of landscape scale variation in mid pleistocene hominin behavior is limited most of our understanding derives from a few localities in eastern africa consequently we know very little about hominin landscape use outside this region despite the fact that mid pleistocene hominins occupied other climatic zones including temperate and mid latitude ecosystems the winter rainfall zone in south africa represents one of the world s most diverse ecosystems although mammal diversity is relatively low in these habitats this is compensated by the tremendous floral diversity buried pleistocene land surfaces in the region provide an opportunity to understand how humans adapted to this unique mid latitude environment prior to the last interglacial the dunefield locality of elandsfontein in the western cape of south africa is one of the richest known paleontological and archeological sites in southern africa it records details of landscape formation history enabling detailed reconstruction of the ancient environments in this winter rainfall dominated region this dunefield archive allows us to posit the taphonomic history of newly excavated archaeological material and previously collected specimens here we provide initial results from the excavation and geological analysis of the elandsfontein dunefield and provide new insights into the formation history of the archaeological and paleontological deposits this includes indications of multiple pedogenic intervals and groundwater table fluctuations the combination of geological paleontological and archaeological data provides a framework for evaluating how hominins interacted with the unique ecosystems of the cape floral region of south africa during the mid pleistocene

diagenesis provenance and reservoir quality of triassic tagi sandstones from ourhoud field berkine ghadames basin algeria the triassic tagi trias argilo greseux inferieur fluvial sandstones are the main oil reservoirs in the berkine basin algeria nonetheless their provenance and diagenesis and their impact on reservoir quality are virtually unknown samples from the ourhoud field representing the lower middle and upper tagi subunits were studied using a combination of petrographic mineralogical and geochemical techniques the lower tagi sandstones have an average framework composition of q98 3f0 6r1 1 and 95 of the quartz grains are monocrystalline by contrast the middle upper tagi sandstones have an average framework composition of q88 3f9 8r1 9 and 79 7 of the quartz grains are monocrystalline the lower tagi quartz arenites derived from paleozoic siliciclastic rocks whereas the middle upper tagi subarkoses originated mainly from metamorphic terrains this change in provenance is a potential criterion for correlation within the tagi also this change has contributed to the significantly different diagenetic paths followed by the lower tagi quartz arenites and the middle upper tagi subarkoses grain coating illitic clays are abundant in the lower tagi where they exert a critical control on reservoir quality these clays are interpreted as pedogenic and or infiltrated in origin and to have had in part smectitic precursors shallow burial fe dolomite cementation was favored in the downthrown block of the field bounding fault where it contributed to the poor reservoir quality magnesite siderite cements are multiphase the earliest generation is composed of fe rich magnesite that precipitated during shallow burial from hypersaline fluids with high mg ca ratios probably refluxed residual brines associated with the liassic evaporites later magnesite siderite generations precipitated during deeper burial from waters with progressively higher fe mg ratios authigenic vermicular kaolin largely consists of dickite that replaced previously formed kaolinite dickitization was followed by late stage illitization related to the dissolution of detrital and authigenic k feldspar quartz the most abundant cement was mainly sourced by the pressure or clay induced dissolution of detrital quartz and is a critical factor controlling the reservoir quality overall quartz cement is more abundant in the lower tagi than in the middle upper tagi and this increase correlates with a decrease in average porosity within the lower tagi quartz cement abundance is stratigraphically very variable which is in part related to facies controlled variations in grain coating clay resulting in major vertical variations in reservoir quality anhydrite and barite cements postdate quartz overgrowth the sulfate necessary for their formation was likely sourced by deep subsurface dissolution of late triassic liassic evaporites

aeolian granule ripple deposits namibia granule ripples are a common feature of most dunefields yet they have seldom been recognized in ancient deposits although granule ripples are common in erosional settings such as windward slopes of dunes or scour surfaces in interdunes they nevertheless migrate laterally and leave distinctive deposits that can be recognized in ancient rocks these deposits have characteristics of type b sand sheet deposits including poured in texture curving ripple trough tangential coarse grained foresets irregular silty layers well sorted coarse and fine layers either horizontal or within foresets and fine layers in ripple troughs wind tunnel experiments suggest that under low velocity wind conditions granule ripples grow to a significant degree as parasites dependent on saltation of fine sand grains whose impact moves the larger grains of the granule ripple although the depositional surface of granule ripples is commonly coated with a layer of coarse grains this is in most places only a few grains thick underlying deposits commonly have a poorly sorted or poured in texture this texture results from an admixture of fine grains that fall among the spaces between the larger grains during deposition

stress gradient coupling in glacier flow iv effects of the t term the t term in the longitudinal stress equilibrium equation for glacier mechanics a double y integral of 2t xy x 2 where x is a longitudinal coordinate and y is roughly normal to the ice surface can be evaluated within the framework of longitudinal flow coupling theory by linking the local shear stress t xy at any depth to the local shear stress t b at the base which is determined by the theory this approach leads to a modified longitudinal flow coupling equation in which the modifications deriving from the t term are as follows 1 the longitudinal coupling length i is increased by about 5 2 the asymmetry parameter is altered by a variable but small amount depending on longitudinal gradients in ice thickness h and surface slope 3 there is a significant direct modification of the influence of local h and on flow which represents a distinct driving force in glacier mechanics whose origin is in pressure gradients linked to stress gradients of the type t xy x for longitudinal variations in h the t force varies as d 2h dx 2 and results in an in phase enhancement of the flow response to the variations in h describable for sinusoidal variations by a wavelength dependent enhancement factor for longitudinal variations in the force varies as d dx and gives a phase shifted flow response although the t force is not negligible its actual effect on t b and on ice flow proves to be small because it is attenuated by longitudinal stress coupling the greatest effect is at shortest wavelengths 2 5h where the flow response to variations in h does not tend to zero as it would otherwise do because of longitudinal coupling but instead because of the effect of the t force tends to a response about 4 of what would occur in the absence of longitudinal coupling if an effect of this small size can be considered negligible then the influence of the t term can be disregarded it is then unnecessary to distinguish in glacier mechanics between two length scales for longitudinal averaging of t b one over which the t term is negligible and one over which it is not longitudinal flow coupling theory also provides a basis for evaluating the additional datum state effects of the t term on the flow perturbations u that result from perturbations h and from a datum state with longitudinal stress gradients although there are many small effects at the 1 level none of them seems to stand out significantly and at the 10 level all can be neglected the foregoing conclusions apply for long wavelengths h for short wavelengths h effects of the t term become important in longitudinal coupling as will be shown in a later paper in this series

the holocene history of the white sands dune field and influences on eolian deflation and playa lakes abstract white sands national monument is the world s largest gypsum dune field the dunes are downwind of a 20 m deep 19 km wide deflation basin containing large playa lakes today the gypsum sand is derived from the edge of the deflation basin next to the dune field rather than the alkali flat and playa lakes where gypsum crystals are forming three erosional shorelines mark wetter episodes when playa lakes formed in the deflation basin successively younger shorelines formed within older ones as the basin deepened but was not widened by deflation the modern shoreline is forming around lake lucero playa the oldest shoreline termed l1 is degraded and formed near the pleistocene holocene transition deflation from the l1 to the l2 shoreline cut through pleistocene bedded evaporites and probably marks initiation of the dune field this event was before 5840 yr bp based on radiocarbon dates this reinforces an evolving consensus that episodes of deflation have characterized desert basins in the southwestern united states regional deflation events have been dated at 7000 and 4000 yr bp the shorelines imply the white sands dune field was created in short episodes and the modern dune field may not represent conditions during expansion of the dune sea

palaeozoic gas charging in the ahnet timimoun basin algeria the ahnet timimoun basin southern algeria contains significant gas reserves expelled from originally oil prone silurian and frasnian shales the gas is reservoired in devonian and carboniferous clastics in inversion anticlines formed primarily during the hercynian orogeny integration of organic and inorganic geochemical techniques such as afta zfta fluid inclusion analysis vitrinite and chitinizoan reflectance is entirely consistent with gas generation 300 30my immediately prior to or synchronous with the hercynian orogeny data from gas fields has shown the remobilisation of gas during post hercynian tectonics a open quotes two event close quotes heating cooling history is proposed 1 maximum burial and palaeotemperature at ca 300 30my prior to or synchronous with hercynian uplift and cooling 2 cooling from a secondary peak lower than maximum palaeotemperature at ca 30 60my following creataceous burial calibrated thermal modelling indicates that palaeozoic source rocks were heated above 200 degrees c in the late carboniferous such high temperatures are consistent with the widespread occurrence of pyrophyllite in silurian shales two end member thermal models can account for the observed maturities the first is a constant high pre hercynian heat flow which rapidly decreases during hercynian uplift to remain at present day values of 50 75mw m sup 2 gas expulsion in this case commences much earlier thanmore trap formation the second is open quotes normal close quotes heat flow of ca 50mw m sup 2 until ca 310my with a rapid increase at ca 290my followed by an equally rapid drop to constant present day values in this model petroleum generation and expulsion is late in relation to structuring less

predicting aeolian sand transport revisiting the white model the derivation and history of the frequently cited aeolian transport model of white are considered in light of the continued replication of an error in the original expression the error may have escaped notice because the expression is still dimensionally correct and it yields predictions that appear reasonable in comparison with both the predictions of other models with field data the incorrect expression has come to be identified as a distinct model however the correct formulation of the white model is in fact a rearrangement of the kawamura model with a slightly smaller c 6 empirical coefficient 1997 john wiley sons ltd

the initiation of particle movement by wind when air blows across the surface of dry loose sand a critical shear velocity fluid threshold ut must be achieved to initiate motion however since most natural sediments consist of a range of grain sizes fluid threshold for any sediment cannot be defined by a finite value but should be viewed as a threshold range which is a function of the size shape sorting and packing of the surface sediment in order to investigate the initiation of particle movement by wind a series of wind tunnel tests was carried out on a range of pre screened fluvial sands and commercially available glass beads with differing mean sizes and sorting characteristics a sensitive laser monitoring system was used in conjunction with a high speed counter to detect initial grain motion and to count individual grain movements test results indicate that when velocity is slowly increased over the sediment surface the smaller or more exposed grains are first entrained by the fluid drag and lift forces either in surface creep rolling or in saltation bouncing or hopping downwind as velocity continues to rise larger or less exposed grains may also be moved by fluid drag on striking the surface saltating grains impart momentum to stationary grains this impact may result in the rebound of the original grain as well as the ejection of one or more stationary grains into the air stream at shear velocities lower than that required to entrain a stationary particle by direct fluid pressure as a result there is a cascade effect with a few grains of varying size initially moving over a range of shear velocities the fluid threshold range and setting in motion a rapidly increasing number of grains results of the tests showed that the progression from fluid to dynamic threshold based on grain movement can be characterized by a power function the coefficients of which are directly related to the mean size and sorting characteristics of the sediment

transport and mixing of eolian sand from local sources resulting in variations in grain size in a gypsum dune field white sands new mexico usa abstract the white sands dune field new mexico usa provides a unique opportunity to study sources and eolian transport of sand end member mixing analysis provides unbiased correlation of the grain size distributions of populations that mix sands from four different local source surface types textural differences between sources allow local transport paths to be deduced in total 1214 surface samples from 10 dunes and 2 downwind oriented transects were collected neither elevation on the dune lee or stoss location nor distance downwind correlated with mean grain size coarsest 10 d90 or sorting instead grain size distributions are controlled by mixing of locally sourced sand populations adjacent dunes can have different mean grain sizes resulting from different local source populations local within dune and between dune variability resulting from different sand sources dominates any larger scale trends across and within dunes four sand populations are identified based on microscopically observable differences in grain size shape and angularity each correlates with high loading of a different statistical factor derived from end member mixing analysis end member 1 em1 correlates with well sorted populations of finer grained equant rounded sands em2 correlates with samples that contain moderately sorted populations containing angular blades and crystal aggregates associated with erosional interdunes em3 is associated with samples of moderately to poorly sorted fine grained sand containing fine sand sized gypsum needles collected from areas of vegetated interdunes and em4 is associated with moderately well sorted coarse and very coarse grained sands collected from granule ripples these results suggest that downwind mixing of different populations and segregation by different depositional processes influence grain size distributions in the dune field rather than by dune scale or erg scale transport and sorting

barchan parabolic dune pattern transition from vegetation stability threshold 1 many dune fields exhibit a downwind transition from forward pointing barchan dunes to stabilized backward pointing parabolic dunes accompanied by an increase in vegetation a recent model predicts this pattern transition occurs when dune surface erosion deposition rates decrease below a threshold of half the vegetation growth rate we provide a direct test using a unique data set of repeat topographic surveys across white sands dune field and find strong quantitative support for the model threshold we also show the threshold hypothesis applied to a barchan dune results naturally in its curvature inversion as the point of threshold crossing progresses from the horns to the crest this simple general threshold framework can be an extremely useful tool for predicting the response of dune landscapes to changes in wind speed sediment supply or vegetation growth rate near the threshold a small environmental change could result in a drastic change in dune pattern and activity

variation in particle size distribution over a small dune the variation in particle size distribution for a sequence of sand samples collected from the surface of a small barchanoid dune along a sampling line parallel to the windflow is studied by means of the hyperbolic distribution the modal or typical value of the log size increases linearly with distance from the windward foot to the crest of the dune and falls off steeply on the lee side the spread of the log size distribution locally near the modal value is roughly constant on the windward side but decreases significantly down the slip face two other parameters of the four parameter hyperbolic distribution can be considered largely constant a comparison of the log size distributions for heavy and light mineral grains based on a selected set of the samples showed that heavy and light minerals follow different hyperbolic patterns however the difference in modal size of the distributions for the two types of minerals was virtually constant among the samples

pattern analysis of dune field parameters as a basic form of pattern analysis the parameters of dune spacing defect density crest orientation and crest length are measured from remote images and treated statistically for dunes at white sands in new mexico the algodones in california the agneitir in mauritania and the namib in namibia statistical populations are identified from frequency plots of dune spacing and crest length field scale calculations of defect density and rose diagrams of crest orientation single populations characterize simple dune fields white sands whereas multiple populations characterize compound complex dunes algodones namib and complex dune fields agneitir as time increases dune fields show an increase in dune spacing and crest length a decrease in defect density more tightly clustered crest orientation and a reduction in the variance associated with measurements of these parameters the results are consistent with models of dune fields as self organizing complex systems in which a characteristic pattern emerges as a function of constructional time because pattern evolution is a function of time it may be possible to use pattern analysis to augment current methods of age determination statistically defined populations can be used in geomorphic backstripping to unstack generations of simple patterns that give rise to complex patterns and to reconstruct each generation in terms of construction time and palaeo wind regime copyright 2006 john wiley sons ltd

saltating particles over flat beds measurements of ejection and impact velocities trajectory lengths and maximum rise heights of sand grains median diameters 118 and 188 m in saltation over a flat sand bed in a wind tunnel have been obtained from the digitization of multiple image photo graphs the mean angle of ejection is found to be about 30 o from the horizontal rather than 90 o with mean vertical ejection velocity of about 2u where u is the friction velocity trajectories of saltating grains have been computed using the measurements of the initial ejection velocities and the mean velocity profile of the air flow

equivalence of abrupt grain size transitions in alluvial rivers and eolian sand seas a hypothesis gravel bed rivers organize to a threshold state in which the fluid stress during dominant flow conditions slightly exceeds the critical value these channels change fairly abruptly to sand dominated beds facilitated by a paucity of sediment in the range of 1 10 mm eolian sediments are also bimodal sand seas often transition rapidly downwind to silty loess deposits in this paper we hypothesize that both transitions result from the same processes 1 production of fine sediment by spallation from saltation abrasion of coarse grains and 2 segregation of fine and coarse modes by suspension and saltation transport respectively we demonstrate that gravel rivers and eolian dune fields are dynamically equivalent in terms of threshold transport shields stress and collision dynamics stokes number while abrasion may not be the dominant cause of downstream fining of fluvial gravels the reduction of collision efficiency with grain diameter sets a lower limit on gravel size leading to a grain size gap the collision efficiency hypothesis potentially reconciles conflicting studies on the importance of sorting and lithology in downstream fining shields and stokes similitude in rivers and deserts may be used to design appropriately scaled experiments to test and refine our hypothesis

a glacier flow model incorporating longitudinal deviatoric stresses le modele d ecoulement de glacier developpe introduit les effets de la composante longitudinale du deviateur des contraintes dans les equations les hypotheses sous jacentes peuvent etre appliquees au developpement de modeles pour des cas de figures variees ici elles sont envisagees pour des glaciers glissant en ecoulement plan a l etat stationnaire et non stationnaire le modele se ramene a une generalisation adequate de la theorie pseudo hydrostatique en ecoulement plan si la contrainte longitudinale deviatrice est negligee devant la contrainte de cisaillement basale

winds and sand movements in the namib sand sea wind regimes in the vicinity of the namib sand sea are high energy unimodal near the coast becoming bimodal or complex inland there is an overall decrease in wind energy and sand transport rates from south to north and west to east such that sand moves from coastal and southern source areas to accumulate in the northern and central parts of the sand sea such a pattern can explain much of the observed spatial variability in dune types sizes and sediment characteristics and lends support to a climatic model of sand sea formation in this region seasonal and daily cycles of wind velocity and direction give rise to episodic sand transport most of which is generated by winds of moderate velocity and frequency

lower silurian hot shales in north africa and arabia regional distribution and depositional model abstract lowermost silurian organic rich hot shales are the origin of 80 90 of palaeozoic sourced hydrocarbons in north africa and also played a major role in petroleum generation on the arabian peninsula in most cases the shales were deposited directly above upper ordovician peri glacial sandstones during the initial early silurian transgression that was a result of the melting of the late ordovician icecap deposition of the main organic rich shale unit in the north african arabian region was restricted to the earliest silurian rhuddanian stage acuminatus atavus and probably early cyphus graptolite biozones during this short period 1 2 m y a favourable combination of factors existed which led to the development of exceptionally strong oxygen deficiency in the area in most countries of the study area the post rhuddanian silurian shales are organically lean and have not contributed to petroleum generation the distribution and thickness of the basal silurian hot shales have been mapped in detail for the whole north african region using logs from some 300 exploration wells in libya tunisia algeria and morocco in addition all relevant accessible published and unpublished surface and subsurface data of the lower silurian shales in north africa and arabia have been reviewed including sedimentological biostratigraphic and organic geochemical data the lowermost silurian hot shales of northern gondwana are laterally discontinuous and their distribution and thickness were controlled by the early silurian palaeorelief which was shaped mainly by glacial processes of the late ordovician ice age and by pan african and infracambrian compressional and extensional tectonism the thickest and areally most extensive basal silurian organic rich shales in north africa occur in algeria tunisia and western libya while on the arabian peninsula they are most prolific in saudi arabia oman jordan and iraq the hot shales were not deposited in egypt which was a large palaeohigh at that time the depositional model presented may help in better understanding the source potential of the basal silurian shales in less explored regions of north africa and arabia including morocco northern niger and the kufra basin in southeast libya

a simple universal equation for grain settling velocity abstract a new equation is presented for sediment fall velocity as a function of grain diameter for given values of fluid viscosity and fluid and solid density sediment fall velocity is a fundamental parameter in the modeling and interpretation of fluviatile and coastal deposition the equation applies to the entire range of viscous to turbulent conditions and its simple explicit form makes it easy to use in computer models and other applications in sedimentology geomorphology and engineering the equation is derived from dimensional analysis and converges on stokes law for small grains and a constant drag coefficient for large grains its two physically interpretable parameters are easily adjusted for shape effects or for the use of sieve diameter rather than nominal grain diameter it gives a close fit to published and new experimental data for both quartz sand and low density materials with no more error than previous equations of more complicated form

the effects of eolian sorting and abrasion upon the shapes of fine quartz sand grains abstract grain shape and surface texture analyses were conducted on fine quartz sand grains from north padre island and the south texas eolian sand sheet the purpose was to determine the effects of eolian shape sorting and abrasion upon the shapes of eolian sand grains two conclusions are reached the first is that the wind preferentially transports spherical rounded sand grains and this can result in the isolation and accumulation of such grains in an eolian deposit after a relatively short transport distance the second is that eolian abrasion can greatly increase the roundness of sand grains during their transport over a distance of less than 160 km abrasion does not however produce any regular downwind change in sand grain sphericity

erosion profiles due to particles entrained by wind application of an eolian sediment transport model erosion by impacts of particles entrained in the wind scales with the kinetic energy of impacting grains a recent model of eolian sediment transport permits calculation of expected erosion patterns in objects of simple geometry the modeling of vertical erosion profiles in turn provides an exacting test of the transport model realistic distributions of liftoff velocities for saltating grains give rise to kinetic energy flux profiles characterized by a strong maximum as much as 1 4 m above the bed during strong winds kinetic energy flux due to suspended grains also peaks above the bed the height and strength of the maxima depend very strongly on the grain size distribution as grain size diminishes increased particle deflection by the air flow around an obstacle reduces delivery of kinetic energy to the surface for both saltating and suspended grains kinetic energy flux scales with the fifth power of the wind shear velocity erosion profiles in man made obstacles are well modeled with only slight modification of the saltation model to account for the relatively high elasticity of actual deflationary surfaces lending considerable support for the model the modeling also elucidates several aspects of ventifaction as small ventifacts 1 2 m in diameter and the lower portions of larger ones are predominantly eroded by saltating grains upwind surface irregularities are quickly damped leading to facets that dip upwind and sharply truncate lee surfaces sandblasting by suspended grains dominates along the upper portions of large ventifacts where grain paths are partially deflected by the obstacle and may lead to radially symmetrical flutes and grooves

a one dimensional finite element model for two dimensional glacier flow on presente une description de la reduction des equations d equilibres 2d vers une forme 1d a travers la methodologie de kantorovich et on traite un exemple qui demontre la capacite du modele numerique a fournir des solutions en accord avec celles obtenues par les modeles a elements finis 2d

on the formation of desert loess sequences of quartz rich coarse 20 63 m silt occur in many low and midlatitude unglaciated arid and semiarid areas and have been termed desert loess the processes by which these deposits are generated have been debated for decades all hypotheses to explain their origin seek to provide mechanisms for the generation of silt sized material without glacial grinding which is the main process involved in the production of coarse silt at high latitudes possible mechanisms for the formation of coarse silt in arid regions include derivation from preexisting siltstones mechanical weathering of silicate rocks and abrasion of sand grains in active dune environments during intense transport events examination of the characteristics of desert loess and field and laboratory experiments to assess the role of dune areas as a source of coarse silt by abrasion and or resuspension of residual fines suggests that many loess sequences are dominated by locally derived coarse silt improvements in the characterization of desert loess particle size mineralogy and geochemistry are needed however to identify sources and sinks of coarse silt especially when combined with climatic back trajectory analysis properly scaled experiments and modeling of particle collisions will also help to better quantify the effectiveness of abrasion in the generation of coarse silt in support of field observations

quantifying the significance of abrasion and selective transport for downstream fluvial grain size evolution it is well known that pebble diameter systematically decreases downstream in rivers the contribution of abrasion is uncertain in part because 1 diameter is insufficient to characterize pebble mass loss due to abrasion and 2 abrasion rates measured in laboratory experiments cannot be easily extrapolated to the field a recent geometric theory describes abrasion as a curvature dependent process that produces a two phase evolution in phase i initially blocky pebbles round to smooth convex shapes with little reduction in axis dimensions then in phase ii smooth convex pebbles slowly reduce their axis dimensions here we provide strong evidence that two phase abrasion occurs in a natural setting by examining downstream evolution of shape and size of thousands of pebbles over 10 km in a tropical montane stream the geometric theory is verified in this river system using a variety of manual and image based shape parameters providing a generalizable method for quantifying the significance of abrasion phase i occurs over 1 km in upstream bedrock reaches where abrasion is dominant and sediment storage is limited in downstream alluvial reaches where phase ii occurs we observe the expected exponential decline in pebble diameter using a discretized abrasion model the so called box equations with deposition we deduce that abrasion removes more than one third of the mass of a pebble but that size selective sorting dominates downstream changes in pebble diameter overall abrasion is the dominant process in the downstream diminution of pebble mass but not diameter in the studied river with important implications for pebble mobility and the production of fine sediments

stress gradient coupling in glacier flow iii exact longitudinal equilibrium equation the vertically integrated exact longitudinal stress equilibrium equation of budd 1970 is developed further in such a way as to yield an equation that gives explicitly and exactly the contributions to the basal shear stress made by surface and bed slope surface curvature longitudinal stress deviators and longitudinal stress gradients in a glacier flowing in plane strain over a bed of longitudinally varying slope with this exact equation questions raised by various approximate forms of the longitudinal equilibrium equation can be answered decisively and the magnitude of errors in the approximations can be estimated to first order in the angle that describes fluctuations in the surface slope from its mean value the exact equilibrium equation reduces to 1 2sin 2 b pghsin 2g t b k where g and t are the well known stress deviator gradient and variational stress terms k is a longitudinal curvature term and b is a basal drag term that contributes a resistance to sliding across basal hills and valleys except for t these terms are expressed in simple form and evaluated for practical situations the bed slope relative to the mean slope is not assumed to be small which allows the effects of bedrock topography to be determined particularly through their appearance in the b term

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

white sands dune field new mexico age dune dynamics and recent accumulations abstract the white sands dune field situated within the tularosa basin in southern new mexico is thought to have been largely derived by a stepwise progressive deflation of pleistocene lake otero strata with the onset of regional aridity optically stimulated luminescence osl dating of samples from a core that penetrated the gypsum accumulation of the dune field confirm a time of origin at 7000 yr dune sediment is characterized as lagged influx from previously stored lake otero sediment and contemporaneous influx derived from subsequent playas sediment became available for aeolian transport and dune field construction because of a falling water table driven by the regional aridity the current dune field is primarily a wet aeolian system in which the behavior of the accumulation surface over time is a function of the water table although surface cementation by gypsum also imparts aspects of a stabilizing system dune crests are oriented borderline transverse to the annual transport resultant and a monitored dune and its cross strata show that transverse winds from the sw during the late winter and spring account for most of the crest normal migration but a significant along crest component of migration of dune sinuosity occurs with fall and winter winds from the nw and n that strike the crests obliquely trenches across three interdune areas show dune sets and interdune strata climbing at about 0 1 depth of interdune scour increases with interdune streamwise length which acts to enhance the probability of dune cross strata deflation during arid years but also increases the probability of interdune accumulation because the water table shows a net rise over time within the trenches dune sets show a bundling of foresets between reactivation surfaces interpreted as annual cycles that reveal lee face reworking by the oblique nw and n winds and slipface progradation fostered by transverse winds from the sw a progression from grainflow dominated to wind ripple dominated cross strata within a single set revealed in the longest trench is consistent with along crest migration of dune sinuosity interdune laminations show light dark couplets interpreted as annual varves reflecting dry wet portions of the year the number of laminations preserved in vertical sections across an interdune area shows that the record is incomplete and endorses the interpretation of periods of interdune deflation a calculation of the current accumulation rate as based upon the climbing strata within the trenches is significantly greater than the long term accumulation rate determined by osl dates taken from the core in addition to compaction and possible dissolution of gypsum as partial explanations the long term accumulation rate may reflect significant variation in the accumulation rate over time short term deflation of accumulations on a regular basis and or the presence of unconformities within the gypsum body

active sand seas and the formation of desert loess abstract loess is a widespread eolian deposit dominated by coarse silt and very fine sand quartz grains 20 80 m that serve as an important archive of information on quaternary climate change despite the intensive research during the past century on desert loess formation its origin remains poorly understood and is still a fundamental problem in sedimentology the ongoing debate is focused on the first stage of loess formation the production of coarse silt quartz grains although laboratory experiments indicate the potential of dunes with their abundant quartz sand grains as a primary source for generating coarse silt grains this concept has been generally rejected because field based evidence of abrasion from sand grains eolian abrasion is scarce here we have adopted a global view of the sand sea loess association and examine in detail several well known late pleistocene loess regions in different subtropical deserts africa the middle east and arabia this analysis demonstrates that all these loess regions are located downwind of adjacent sand seas together with evidence of mineralogical similarity between the loess and the sand dunes and the contemporaneous deposition of the loess and movement of the dunes these observations suggest that sand seas are an important source for desert loess because there is only limited silt storage in sand dunes we postulate that the silt grains comprising most of the loess are not reworked from the dunes but are generated probably through active eolian abrasion of the sand grains under past climates characterized by intensified winds as a result the role of sand dunes and eolian abrasion in the formation of desert loess may be more important than previously thought

physical abrasion of mafic minerals and basalt grains application to martian aeolian deposits abstract sediment maturity or the mineralogical and physical characterization of sedimentary deposits has been used to identify sediment sources transport medium and distance weathering processes and paleoenvironments on earth mature terrestrial sands are dominated by quartz which is abundant in source lithologies on earth and is physically and chemically stable under a wide range of conditions immature sands such as those rich in feldspars or mafic minerals are composed of grains that are easily physically weathered and highly susceptible to chemical weathering on mars which is predominately mafic in composition terrestrial standards of sediment maturity are not applicable in addition the martian climate today is cold and dry and sediments are likely to be heavily influenced by physical weathering rather than chemical weathering due to these large differences in weathering processes and composition martian sediments require an alternate maturity index this paper reports the results of abrasion tests conducted on a variety of mafic materials and results suggest that mature martian sediments may be composed of well sorted well rounded spherical polycrystalline materials such as basalt volcanic glass is also likely to persist in a mechanical weathering environment while more fragile and chemically altered products are likely to be winnowed away a modified sediment maturity index is proposed that can be used in future studies to constrain sediment source paleoclimate mechanisms for sediment production and surface evolution this maturity index may also provide insights into erosional and sediment transport systems and preservation processes of layered deposits

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

shape effects in aeolian grain transport wind tunnel experiments are reported the results of which show that grain transport rate is responsive to characteristic grain shape it is relatively less sensitive to changes of sorting unless these involve the fraction smaller than 50 m a cubic relationship between transport rate and velocity gradient appears to be valid only for well worked natural grains having high sphericity

spatial grain size sorting in eolian ripples and estimation of wind conditions on planetary surfaces application to meridiani planum mars the landscape seen by the mars exploration rover mer opportunity at meridiani planum is dominated by eolian wind blown ripples with concentrated surface lags of hematitic spherules and fragments these ripples exhibit profound spatial grain size sorting with well sorted coarse grained crests and poorly sorted generally finer grained troughs these ripples were the most common bed form encountered by opportunity in its traverse from eagle crater to endurance crater field measurements from white sands national monument new mexico show that such coarse grained ripples form by the different transport modes of coarse and fine grain fractions on the basis of our field study and simple theoretical and experimental considerations we show how surface deposits of coarse grained ripples can be used to place tight constraints on formative wind conditions on planetary surfaces activation of meridiani planum coarse grained ripples requires a wind velocity of 70 m s at a reference elevation of 1 m above the bed from images by the mars orbiter camera moc of reversing dust streaks we estimate that modern surface winds reach a velocity of at least 40 m s and hence may occasionally activate these ripples the presence of hematite at meridiani planum is ultimately related to formation of concretions during aqueous diagenesis in groundwater environments however the eolian concentration of these durable particles may have led to the recognition from orbit of this environmentally significant landing site

the role of diagenesis and depositional facies on pore system evolution in a triassic outcrop analogue se spain abstract this study aims at unravelling the diagenetic history and its effect on the pore system evolution of the triassic redbeds exposed in se spain tibem 1 an outcrop analogue of the tagi trias argilo greseux inferieur reservoir berkine ghadames basin algeria similar climatic base level and tectonic conditions of aforementioned alluvial formations developed analogue fluvial facies stacking patterns furthermore interplay of similar detrital composition and depositional facies in both formations resulted in analogue early diagenetic features petrographic observations indicate lithic subarkosic floodplain facies and subarkosic braidplain facies compositions which are considered suitable frameworks for potential reservoir rocks primary porosity is mainly reduced during early diagenesis through moderate mechanical compaction and formation of k feldspar overgrowth gypsum dolomite and phyllosilicate cements early mesodiagenesis is testified by low chemical compaction and quartz cementation telodiagenetic calcite filling fractures and k feldspar dissolution determined the final configuration of analysed sandstones mercury injection capillary pressure technique reveals overbank deposits in the floodplain as the least suitable potential reservoirs because of their lowest open porosity op k k 95 md and greater thickness and lateral continuity so being considered the best potential reservoir the accurate estimation of tibem microscale attributes can provide important input for appraisal and enhanced oil recovery performance in tagi and in others reservoirs consisting on similar fluvial sandy facies

an integrated model for dune morphology and sand fluxes on mars the transport and deposition of sand is the most prevalent agent of landscape modification on mars today with fluxes comparable to some sand dunes on earth until now the relationship between sand flux and dune field morphology has been poorly constrained by tracking dune movement over 10 km long dune fields in herschel crater and nili patera representative of many dune fields on mars we find a downwind flux decrease that correlates with a sequence of changing morphology from barchans to barchanoids and seifs longitudinal dunes to isolated dome dunes and ending with sand sheets we show empirical consistency with atmospheric internal boundary layer ibl theory which can describe these broad flux and morphology changes in martian dune fields deviations from ibl flux predictions are from wind streamline compressions up slopes leading to a speedup effect by establishing a dune field morphology type example and correlating it with measured and predicted flux changes we provide an integrated morphology and flux model that can be applied to other areas of mars and be used to infer paleo environmental conditions from preserved sandstone

potassium argon timing of episodic mica and illite crystallization in highly indurated hassi messaoud algeria hydrocarbon bearing sandstones nanometer to micrometer mica and illite separates of indurated cambrian and ordovician oil bearing sandstones from the hassi messaoud field algeria were extracted x rayed observed by scanning and transmission electron microscopy and k ar dated electron microscope observations revealed typical euhedral shapes for the mica to illite particles of most size fractions almost no odd shaped detrital crystals were detected the combined results document several generations of mineralogical and morphological identical mica to illite crystals that could not be differentiated by the traditional identification methods illite and mica genesis was multiphased with crystallization episodes at 340 10 ca middle mississippian 280 10 ma ca early permian and 170 10 ma ca middle jurassic younger than the stratigraphic age of the host rocks which is incompatible with a detrital origin the two older mica ages confirm that the hydrocarbon generation and emplacement had to start after the variscan tectonothermal event and before exhumation of the meta sediments the younger k ar ages at 135 to 110 ma ca early cretaceous relate to further crystallization episodes whereas those at circa 295 265 and 210 ma probably correspond to variable mixtures of the older and younger mica to illite end members three average k ar values are statistically significant the oldest at 340 10 ma corresponds to the start of the variscan tectonic activity and the intermediate at 280 10 ma sets its end both episodes probably modifying the reservoir capacities of the potential hydrocarbon host rocks the ages at 170 10 ma identify a further diagenetic activity characterized by illitization of dickite type precursors in local reservoirs these younger ages could correspond to the hydrocarbon charge into reservoirs which stopped diagenetic illitization at a present day depth of approximately 4000 m 13 000 ft

aeolian dune field pattern boundary conditions aeolian dune field patterns reflect the complex external environment within which the pattern evolves these external environmental controls are the boundary conditions on aeolian dune field pattern formation the influences of boundary conditions such as wind regime and sediment supply are well known however boundary conditions represent a relatively unexplored area of dune field pattern formation source area geometry and areal limits are two newly recognized boundary conditions measurements of crest spacing and crest length from satellite images of dune field patterns with point and line source area geometries show an increase in spacing and crest length over distance whereas spacing and crest length in plane sourced patterns emerge equally across the dune field the impact of the size and shape of a dune field on crest spacing is tested using a previously established analytical model model results indicate that the area of a dune field limits the maximum spacing that can occur within a given area and that dune fields that are five times longer in the direction of dune migration than in the crest parallel direction can achieve the greatest spacing within a given area empirical measurements of spacing defect density and dune field area from ten different dune fields ranging over four orders of magnitude in size show that spacing increases and defect density decreases as dune field area increases because the formation of all dune fields involves a wind regime and sediment supply there must be interplay of boundary conditions within the same dune field at different stages of pattern development one boundary condition may dominate over others and with changing climatic eustatic or tectonic parameters boundary conditions may change to modify an existing pattern recognizing boundary condition controls on aeolian dune field pattern formation provides a framework for recognizing the signature of the external environment in which a pattern developed and can be used for reconstructing past dune constructional events and climatic change