early neoproterozoic crustal evolution in northern yili block insights from migmatite orthogneiss and leucogranite of the wenquan metamorphic complex in the nw chinese tianshan abstract the northern part of the yili block located in the northwest of the chinese tianshan extends westward to joint with the aktau junggar domain in kazakhstan and is one of the major continental constituents of the central asian orogenic belt caob the nature and tectonic significance of the basement of the continental domains are important for understanding the continental evolution and geodynamic processes of the caob we investigated the wenquan metamorphic complex wmc that represents the basement and metamorphosed sedimentary cover of the northern yili block the wmc is mainly composed of gneissic s type granite migmatite associated with amphibolite paragneiss micaschist quartzite marble and intruding leucogranitic dykes the migmatites display banded schlieren and ptygmatic structures containing rootless lensoid or layered leucosome and leucocratic sills which are indicative of in situ partial melting of the country rocks and variable degrees of melt migration geochemical data suggest that the protoliths of the orthogneiss and gneissic k granite belong to peraluminous s type granites characterized by wide range of i sr values 0 68324 0 72365 low nd t values 1 9 to 4 4 and two stage nd model ages t md 2 1 56 1 82 ga the s type granites might have derived from both clay poor and clay rich source rocks that are probably meta sedimentary and meta volcanic rocks in the wmc the leucogranites show relatively narrow range of i sr values 0 70273 0 70419 more negative nd t values 6 9 to 9 1 and more consistent t md 2 ages 1 51 1 55 ga the leucosome and leucocratic sills show variable sr and nd isotopic compositions probably due to localized and different degree of partial melting our new zircon shrimp and la icpms u pb ages together with previously published data indicate that the migmatization occurred at 926 909 ma the gneissic s type granites emplaced during 919 862 ma and the leucogranites emplaced in the period of 909 845 ma inherited old zircons were found in all dated samples and may have come from their source rocks association of geochronologically overlapped and genetically linked migmatites gneissic s type granites and leucogranitic rocks is interpreted as the results of anatexis of the upper crustal rocks during an early neoproterozoic tectonic metamorphic event that needs to be further studied in the northern yili block such early neoproterozoic migmatization and s type granitic magmatism are considered as an important episode of reworking and cratonization of the continental crust of the yili and adjacent continental blocks in central asia

origin of co2 rich fluid inclusions in leucosomes from the skagit migmatites north cascades washington usa co2 ch4 fluid inclusions are present in anatectic layer parallel leucosomes from graphite bearing metasedimentary rocks in the skagit migmatite complex north cascades washington petrological evidence and additional fluid inclusion observations indicate however that the skagit gneiss was infiltrated by a water rich fluid during high temperature metamorphism and migmatization co2 rich fluid inclusions have not been observed in skagit metasedimentary mesosomes or melanosomes meta igneous migmatites or unmigmatized rocks and are absent from subsolidus leucosomes in metasedimentary migmatites the observation that co2 rich inclusions are present only in leucosomes interpreted to be anatectic based on independent mineralogical and chemical criteria suggests that their formation is related to migmatization by partial melting although some post entrapment modification of fluid inclusion composition may have occurred during decompression and deformation the generation of the co2 rich fluid is attributed to water saturated partial melting of graphitic metasedimentary rocks by a reaction such as biotite plagioclase quartz graphite al2sio5 water rich fluid garnet melt co2 ch4 the presence of co2 rich fluid inclusions in leucosomes may therefore be an indication that these leucosomes formed by anatexis based on the inferences that 1 an influx of fluid triggered partial melting and 2 some episodes of fluid inclusion trapping are related to migmatization by anatexis it is concluded that a free fluid was present at some time during high temperature metamorphism the infiltrating fluid was a water rich fluid that may have been derived from nearby crystallizing plutons because partial melting took place at pressures of at least 5 kbar abundant free fluid may have been present in the crust during orogenesis at depths of at least 15 km

clinopyroxene composition of ophiolite basalts as petrogenetic indicator abstract a statistical microprobe study has been carried out on augitic clinopyroxene in high ti to low ti metabasalts from various phanerozoic ophiolites a comparison has been made with clinopyroxenes from mid ocean ridge basalts morb oceanic intraplate basalts island arc tholeiites and boninites leaving aside the effect of cooling rate and of fractional crystallization as major factors influencing pyroxene chemistry their composition appears to be essentially related to the type of host magma both the tio2 and the tio 2 al 2 o 3 in clinopyroxenes are positively correlated with the zr y ratios of the host basalts which is a diagnostic parameter insensitive to alteration increasing regularly from low ti to high ti ophiolitic basalts ti also shows a positive correlation and average d ti cpx rock partition coefficients varying from 0 3 0 6 to 1 from low ti to high ti basalts which compare favorably with those from tongan southwest pacific basaltic andesites 0 3 0 6 and morb 0 75 respectively likewise the average k fe mg calculated for clinopyroxene rock pairs varies from 0 20 in low ti basalts to 0 26 in high ti basalts up to 0 33 in the highest ti rocks suggesting a significant dependence of this parameter on magma composition the compositional correspondence of clinopyroxenes from various ophiolitic basalts with those from oceanic actualistic equivalents is shown in various diagrams concomitantly with increasing tio2 and na2o and decreasing sio2 contents clinopyroxenes from very low ti to high ti ophiolitic basalts are best equated with those from boninites basaltic andesites and tholeiites from island arcs and morb respectively accordingly a parallel increase is shown by the substitutional molecules catial2o6 and natisialo6 within each basalt type fractional crystallization processes are reflected in the natisialo6 and fetial2o6 increase and mgcrsialo6 decrease from early to late crystallized clinopyroxenes

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

origin of the skagit migmatites north cascades range washington state stromatic and schlieren type migmatites are a major lithology in the type section of the skagit gneiss complex in the north cascades range of washington state usa migmatite mesosomes are chiefly biotite schist amphibolite and orthogneiss in decreasing order of abundance leucosomes are predominantly leucotrondhjemites with a very limited range of composition that is nearly independent of associated mesosome type melanosomes consisting mainly of biotite and or hornblende garnet are inconsistently developed and absent in places the age of migmatization is not well established but appears to be late cretaceous or early tertiary this is also the age of syntectonic tonalite to trondhjemite intrusives that are predominant in most parts of the skagit complex although temperatures in excess of 700 c and pressures as high as 10 kb occurred there is no evidence for widespread partial melting of the mesosomes with which the migmatites are closely associated mass balance calculations preclude an origin by injection of a silicate melt or hydrothermal fluid unless accompanied by metasomatic replacement reactions mass balance relationships also show that the skagit migmatites could not have formed solely by closed system processes such as partial melting or metamorphic segregation unless the mesosomes present were not the protolith from which the migmatites formed field petrographic and geochemical data indicate that an origin by migmatization of a missing mesosome is quite unlikely the most feasible process of migmatization appears to be infiltration of an aqueous fluid into a metamorphic protolith along fracture or foliation planes this triggers a variable degree of metamorphic segregation or possibly minor partial melting unmixing of leucosomes and melanosomes from the mesosome protolith must be accompanied by metasomatic replacement but the total mass transfer required is only a few wt

pin stripe lamination a distinctive feature of modern and ancient eolian sediments abstract pin stripe laminations are a distinctive feature of modern and ancient eolian sediments in sets of eolian ripple or translatent strata they represent deposition of silt and very fine sand in the troughs of the advancing wind ripples in sets of avalanche strata they probably result from the downward settling of fine sand and silt within the moving avalanche to the interface of moving and unmoving sands wind tunnel experiments suggest that pin stripe laminations can also form in grainfall deposits the textural segregation associated with deposition of the fine layers in most cases leads to early cementation along and near the finest sand and silt comprising the pin stripe lamination the pin stripe effect seen in outcrops is usually due to resistance to weathering along such cemented zones the cementation of the pin stripe laminations can occur early in the history of diagenesis and thus may provide clues to the post depositional history of the rock pin stripe laminations in many instances represent the sequestering of the small population of ultrafine sediment present in most eolian depositional systems they may prove useful in the recognition of ancient eolian sediments

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

origin of k poor leucosomes in a metasedimentary migmatite complex by ultrametamorphism syn metamorphic magmatism and subsolidus processes two types of stromatic leucosomes are identified in metasedimentary rocks from the skagit migmatite complex north cascades washington state u s a both types are trondhjemitic and appear similar in outcrop but although both contain low abundances of ree one type consists of leucosomes that are relatively ree enriched compared to the other and contains 1 small 700 c water rich fluid present inferences about the origin of the above listed mineralogical and fluid inclusion features and modeling of leucosome trace element abundances the second type of leucosome is interpreted to have formed entirely by subsolidus processes e g metamorphic differentiation because these leucosomes lack features consistent with an origin by partial melting k poor tonalitic trondhjemitic leucosomes associated with metasedimentary biotite bearing source rocks may form by water saturated partial melting or by subsolidus processes both general leucosome forming mechanisms may operate at different times during upper amphibolite facies regional metamorphism partial melting may be initiated by syn metamorphic magmatic activity if crystallizing plutons serve as external sources of the water rich fluid necessary for ultrametamorphism in the middle crust during orogenesis large scale migmatite complexes such as the skagit migmatites may form at least in part in response to contact effects of plutonism associated with high grade metamorphism so although migmatite complexes are a volumetrically substantial part of many orogenic belts they may not themselves represent a significant original source of magma for larger scale igneous bodies

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

annual monsoon rains recorded by jurassic dunes pangaea the largest landmass in the earth s history was nearly bisected by the equator during the late palaeozoic and early mesozoic eras modelling experiments and stratigraphic studies have suggested that the supercontinent generated a monsoonal atmospheric circulation that led to extreme seasonality1 2 3 but direct evidence for annual rainfall periodicity has been lacking4 in the mesozoic era about 190 million years ago thick deposits of wind blown sand accumulated in dunes of a vast low latitude desert at pangaea s western margin5 6 7 these deposits are now situated in the southwestern usa here we analyse slump masses in the annual depositional cycles within these deposits which have been described for some outcrops of the navajo sandstone8 twenty four slumps which were generated by heavy rainfall appear within one interval representing 36 years of dune migration we interpret the positions of 20 of these masses to indicate slumping during summer monsoon rains with the other four having been the result of winter storms the slumped lee faces of these jurassic dunes therefore represent a prehistoric record of yearly rain events

significance of interdune deposits and bounding surfaces in aeolian dune sands bounding surfaces and interdune deposits provide keys for detailed interpretations of the development shape type wavelength and angle of climb of aeolian bedforms as well as overall sand sea conditions current alternate interpretations of bounding surfaces require very different but testable models for sand sea deposition two perpendicular traverses of jurassic entrada sandstone utah reveal relations among cross strata first order bounding surfaces and horizontal strata these field relations seem explicable only as the deposits of downwind migrating climbing enclosed interdune basins horizontal strata and dune bodies consisting of superimposed smaller crescentic dunes cross stratified deposits a 1 7 km traverse parallel to the palaeowind direction provides a time transgressive view showing continuous cosets of cross strata first order bounding surfaces and interdune deposits climbing downwind at an angle of a few tenths of a degree changes occur in the angle of climb cross strata structure and interdune deposits these reflect changes in depositional conditions through time a 1 5 km traverse perpendicular to the palaeowind direction provides a view at an instant in geological time showing first order bounding surfaces and interdune deposits forming flat laterally discontinuous lenticular bodies the distribution of interdune sedimentary structures in this traverse is very similar to that of some modern interdune basins such as those on padre island texas hierarchies of bounding surfaces in an aeolian deposit reflect the bedform development on an erg the presence of three orders of bounding surfaces indicates dune bodies consisting of smaller super imposed dunes the geometry of first order bounding surfaces is a reflection of the shape of the inter dune basins second order bounding surfaces originate by the migration of the superimposed dunes over the larger dune body and reflect individual dune shape and type third order bounding surfaces are reactivation surfaces showing stages in the advance of individual dunes the presence of only two orders of bounding surfaces indicates simple dunes modern and entrada interdune deposits show a wide variety of sediment types and structures reflecting deposition under wet damp and dry conditions interdune deposits are probably the best indicators of overall erg conditions and commonly show complex vertical sequences reflecting changes in specific depositional conditions

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

migmatization by metamorphic segregation at subsolidus conditions implications for nd pb isotope exchange abstract we have carried out major and trace element analyses as well as nd and pb isotope measurements on a stromatic migmatite in the phikwe complex botswana in order to assess the effect of high grade metamorphic event leading to migmatization on apparent nd model ages the phikwe complex considered as an entity of the central zone of the limpopo belt underwent a high grade metamorphic event at 2 0 ga leading to migmatite formation migmatites in this area present a very good layering of migmatitic components leucocratic layers were segregated from mafic layers dominated either by hornblende or biotite on a centimeter to millimeter scale accessory minerals such as monazite apatite and zircon were not identified in hornblende rich and quartzo feldspathic layers the combination of petrography and trace element concentrations in all layers indicates that the migmatite may have been formed either by partial melting according to biotite and or hornblende dehydration reactions or by metamorphic segregation at subsolidus conditions ree modeling applying batch fractional and disequilibrium melting equations allows to discriminate between these two processes the modeled ree results of the quartzo feldspathic layers match those observed via metamorphic segregation at subsolidus conditions the end values at 2 0 ga range from 0 3 to 8 9 with lower values in the ree rich migmatitic layers thus the distribution of major and trace elements as well as the scatter nd isotope data are interpreted as reflecting the proportional amount of each mineral in the different migmatitic components the positive end value of hornblende rich layers suggests hornblende crystallization from a mantle derived source and sm nd fractionation at protolith formation age apparent nd model ages ranging from 2 3 to 3 1 ga do not reflect the crustal evolution in southern africa in archean to proterozoic time based on the geochemical features and the pb pb wr isochron at 2 0 ga mass conservation during migmatization can be assumed therefore chemical and nd isotopic compositions of the protolith prior to migmatization can be calculated the results indicate a si and k poor diorite as protolith moreover calculated protolith compositions yield a two point sm nd isochron indicating an age of 2 7 ga and very similar nd model ages of 2 73 2 75 ga those ages constrain the timing of an archean crust formation episode in the phikwe complex

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

effects of atmospheric conditions on wind profiles and aeolian sand transport with an example from white sands national monument quantifying aeolian sand transport rates relies upon the computation of the near surface shear velocity u determined from velocity profiles of the wind while it has been recognized that various conditions such as saltation surface roughness surface slope and atmospheric conditions have an effect on the velocity profile it is commonly assumed that measurements made above the surface will be representative of the near surface shear velocity airflow and temperature data collected over a flat substrate at white sands national monument in new mexico however show the significant effects that atmospheric conditions have on velocity profiles during the day when solar insolation is heating the surface atmospheric conditions in the lowest several metres become unstable resulting in enhanced convection and vertical mixing so that the velocity gradient changes little with height as a result the shear stress in this region of vertical mixing lessens while the near surface shear stress is increased because the higher wind speeds are now nearer the surface at night the near surface atmospheric conditions are stable thereby reducing convection and vertical mixing resulting in stratified airflow and increased shear velocity away from the surface unless this atmospheric effect is accounted for estimates of sediment transport rates may be in error by as much as a factor of 15 times when wind speeds are near threshold velocity at wind speeds approaching 10 ms1 at 5m above the surface this error in computing sediment transport is reduced to a factor of only two to three times and may be within the range of measurement error

wind directions predicted from global circulation models and wind directions determined from eolian sandstones of the western united states a comparison panfish j t and peterson f 1988 wind directions predicted from global circulation models and wind directions determined from eolian sandstones of the western united states a comparison in g kocurek editor late paleozoic and mesozoic eolian deposits of the western interior of the united states sediment geol 56 261 282 wind directions for middle pennsylvanian through jurassic time are predicted from global circulation models for the western united states these predictions are compared with paleowind directions interpreted from eofian sandstones of middle permsylvanian through jurassic age predicted regional wind directions correspond with at least three quarters of the paleowind data from the sandstones the rest of the data may indicate problems with correlation local effects of paleogeography on winds and lack of resolution of the circulation models the data and predictions suggest the following paleoclimatic developments through the time interval studied predominance of winter subtropical high pressure circulation in the late pennsylvanian predominance of summer subtropical high pressure circulation in the permian predominance of summer monsoonal circulation in the triassic and earliest jurassic and during the remainder of the jurassic influence of both summer subtropical and summer monsoonal circulation with the boundary between the two systems over the western united states this sequence of climatic changes is largely owing to paleogeographic changes which influenced the buildup and breakdown of the monsoonal circulation and possibly owing partly to a decrease in the global temperature gradient which might have lessened the influence of the subtropical high pressure circulation the atypical humidity of triassic time probably resulted from the monsoonal circulation created by the geography of pangaea this circulation is predicted to have been at a maximum in the triassic and was likely to have been powerful enough to draw moisture along the equator from the ocean to the west

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

inconsistencies between pangean reconstructions and basic climate controls the supercontinent pangea dominated our planet from the permian into the jurassic paleomagnetic reconstructions have been used to estimate the latitudinal position of pangea during this 100 million year period atmospheric circulation recorded by eolian sandstones in the southwestern united states shows a broad sweep of northeasterly winds over their northernmost extent curving to become northwesterly in the south this evidence is consistent with paleomagnetic reconstructions of the region straddling the equator in the early permian but is at odds with its northward movement to about 20 n by the early jurassic at least one of the following scenarios must be true the latitude based on paleomagnetism is incorrect the interpretation of how winds shaped the dunes is mistaken the basic climate controls in the jurassic were different from those of today or the paleogeographic reconstructions available are insufficient to adequately reproduce the wind fields responsible for dune formation

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

origin and evolution of a migmatite the development of a stromatic migmatite exposed east and southeast of arvika western sweden is described in four stages beginning with the country rock and following evolution through three areas characterized by low medium and high amounts of leucosomes areas l m and h respectively

threshold for particle entrainment into suspension laboratory observations regarding the limit conditions for particle entrainment into suspension are presented a high speed video system was used to investigate conditions for the entrainment of sediment particles and glass beads lying over a smooth boundary as well as over a rough bed the results extend experimental conditions of previous studies towards finer particle sizes a criterion for the limit of entrainment into suspension is proposed which is a function of the ratio between the flow shear velocity and particle settling velocity observations indicate that particles totally immersed within the viscous sublayer can be entrained into suspension by the flow which contradicts the conclusions of previous researchers a theoretical analysis of the entrainment process within the viscous sublayer based on force balance considerations is used to show that this phenomenon is related to turbulent flow events of high instantaneous values of the reynolds stress in agreement with previous observations in the case of experiments with a rough bed a hiding effect was observed which tends to preclude the entrainment of particles finer than the roughness elements this implies that as the ratio between particle and roughness element sizes becomes smaller progressively higher bed shear stresses are required to entrain particles into suspension on the other hand an overexposure effect was also observed which indicates that a particle moving on a smooth bed is more prone to be entrained than the same particle moving on a bed formed by identical particles

an empirical model of aeolian dune lee face airflow airflow data gathered over dunes ranging from 60 m tall complex crescentic dunes to 2 m tall simplecrescentic dunes were used to develop an empirical model of dune lee face airflow for straight crested dunes the nature of lee face flow varies and was found to be controlled by the interaction of at least three factors dune shape the incidence angle between the primary wind direction and the dune brinkline and atmospheric thermal stability three types of lee face flow separated attached and deflected along slope or attached and undeflected were found to occur separated flows characterized by a zone of low speed 0 3o of crestal speed back eddy flow typically occur leeward of steep sided dunes in transverse flow conditions unstable atmospheric thermal stability also favours flow separation attached flows characterized by higher flow speeds up to 84 of crestal speed that are a cosine function of the incidence angle typically occur leeward of dunes that have a lower average lee slope and are subject to oblique flow conditions depending on the slope of the lee face attached flow may be either deflected along slope lee slopes greater than about 20 or have the same direction as the primary flow lee slopes less than about 20 neutral atmospheric thermal stability also favours flow attachment as each of the three types of lee face flow is defined by a range of wind speeds and directions the nature of lee face flow is intimately tied to the type of aeolian depositional process i e wind ripple or superimposed dune migration grainflow or grainfall that occurs on the lee slope and the resulting pattern of dune deposits therefore the model presented in this paper can be used to enhance the interpretation of palaeowind regime and dune type from aeolian cross strata

cyclic cross bedding in the eolian dunes of the sergi formation upper jurassic reconcavo basin inferences about the wind regime abstract the upper jurassic sergi formation displays eolian cyclic cross bedding composed of a regular intercalation of grain flow and wind ripple packages forming 1 3 m thick cycles each cycle is bound by erosive downwind surfaces and it comprises a sequence of lower angle wind ripple laminae followed by steeper grain flow strata that built out upon the wind ripple wedges in the migration direction the cross strata mean vectors dip consistently towards sw these cycles are related to seasonal changes in wind direction the cyclic stratification pattern suggests an alternation of northeasterly transverse winds responsible for slipface evolution and dune migration followed by reverse transversal or oblique winds overall southerly winds these winds caused dune degradation and reworking and led to the formation of reactivation surfaces and the related deposition of wind ripple wedges the wind pattern inferred from the eolian cyclic cross bedding is consistent with the quantitative atmospheric circulation model that indicates that a monsoonal wind regime operated in the low latitudes of gondwana during the late jurassic

high pressure metamorphism in the western cordillera of north america the skagit gneiss a major component of the crystalline core of the north cascades was metamorphosed during a mid cretaceous to early tertiary high p event driven by the collision of the insular and intermontane superterranes maximum pressures recorded by metapelitic rocks are 8 10 kbar at 650 725 c high pressures are also indicated by coexisting staurolite and hornblende in amphibolites in the skagit gneiss and adjacent cascade river schist mineral reactions continued during nearly isothermal decompression from 8 10 kbar to c 3 5 kbar early high p minerals e g kyanite are present as armoured relics in garnet in gneisses that contain sillimanite and cordierite in the groundmass skeletal relics of kyanite are also present in the groundmass of lower grade staurolite bearing schists that contain texturally later cordierite this matrix kyanite may have been preserved as a result of rapid uplift following initial decompression at high temperature these results represent a revision of the metamorphic history of the skagit gneiss which was formerly thought to have experienced only relatively low p barrovian metamorphism qualitative estimates of metamorphic conditions based on stable matrix mineral assemblages result in an underestimation of maximum pressures because mineral reactions continued during decompression geobarometric results for the skagit gneiss are interpreted as evidence for major crustal thickening in the north cascades recognition that pressures of c 9 kbar were attained supports a contractional model for north cascades orogenesis and requires that tectonic syntheses account for the burial of the skagit gneiss protoliths to a depth of c 25 30 km

high resolution stratigraphic architecture and lithological heterogeneity within marginal aeolian reservoir analogues marginal aeolian successions contain different lithological units with variable geometries dimensions and spatial distributions such variations may result in considerable heterogeneity within hydrocarbon reservoirs developed in successions of this type which poses a high risk to their efficient development here such heterogeneity is described and characterized at inter well 1 km scales using two well exposed outcrop analogues of end member marginal aeolian deposits from the permian cedar mesa sandstone and jurassic page sandstone of south central utah usa the sedimentology and stratigraphic architecture of the cedar mesa sandstone was studied in a 1 2 km2 area in the indian creek region of southern utah where the interval consists of interbedded fluvial and aeolian deposits representative of a fluvial dominated erg margin the page sandstone was studied in a 4 3 km2 area near escalante close to the utah arizona border where it consists of interbedded sabkha and aeolian deposits representative of a transitional marine erg margin the three dimensional stratigraphic architectures of both reservoir analogues have been characterized in order to establish the dimensions geometries and connectivity of high permeability aeolian sandstones facies architecture of the aeolian sabkha deposits is characterized by laterally continuous aeolian sandstone layers of relatively uniform thickness that alternate with layers of heterolithic sabkha deposits aeolian sandstones are thus likely to form vertically unconnected but laterally widespread flow units in analogous reservoirs facies architecture in the aeolian fluvial deposits is more complex because it contains alternating intervals of aeolian sandstone and fluvial heterolithic strata both of which may be laterally discontinuous at the studied length scales aeolian sandstones encased by fluvial heterolithic strata may form small isolated flow units in analogous reservoirs although the limited continuity of fluvial heterolithic strata results in vertical connectivity between successive aeolian sandstones in other locations these architectural templates may be used to condition zonation schemes in models of marginal aeolian reservoirs

eolian origin of upper paleozoic sandstones southeastern utah abstract throughout its outcrop in southeastern utah the permian cedar mesa sandstone contains large scale crossbedding and is devoid of marine fossils except for sand size skeletal fragments although this rock unit has been interpreted by several recent workers as shallow marine stratification identical to that documented in modern eolian dunes is ubiquitous in the cedar mesa thin inversegraded laminations within crossbeds represent the unique product of climbing wind ripples this type of stratification is the best evidence for an eolian origin of the cedar mesa such an interpretation is strongly supported by other evidence vertebrate trackways and simple small cylindrical burrows constitute a distinctively nonmarine ichnofauna rhizoliths root casts are abundant at many stratig aphic levels in the underlying rico and hermosa formations sandstones with physical and biogenic sedimentary structures identical to those in the cedar mesa are interbedded with crossbedded limestones with abundant megafossils and marine traces contacts between sandstones and limestones are sharp gradational relationships would be expected if both sandstones and limestones accumulated subaqueously siliciclastic pebbles are present in some of these carbonate rocks suggesting that the well sorted nature of the sandstones cannot be attributed to the unavailability of coarse clasts palcocurrent data from the sandstones and limestones show significantly different vector means and variance southward pinchout of marine carbonates indicates that winds were onshore winnowing of silicicla tics from coarser carbonate detritus created a strong dichotomy in bulk mineralogy between marine and eolian sediments reinterpretation of the sandstones necessitates reappraisal of ideas concerning the mineralogical maturity of these sediments the relative importance of eolian and shallow marine transport processes to the destruction of labile siliciclastics is unknown

ree distribution in some layered migmatites constraints on their petrogenesis abstract the ree distributions in mesosomes neosomes leucosomes and melanosomes of four layered migmatites have been investigated in one example arvika migmatites the ree patterns in adjacent paragneisses the presumed parent rock of the migmatites were also determined ree patterns of neosomes and mesosomes of arvika migmatites are similar to the finegrained layers and coarse grained layers respectively observed in the adjacent paragneiss this is in agreement with the layer by layer paragneiss migmatite transformation model the ree patterns of mesosomes and neosomes indicate that these lithologies may have been closed systems for ree during the formation of the migmatites no indication of metasomatic reactions melt segregation or injection could be detected within the neosomes leucosomes are depleted and melanosomes enriched in ree contents this is interpreted to be due to separation and concentration of accessory minerals monazite epidote allanite zircon sphene apatite garnet into the melanosomes the behaviour of accessory minerals during migmatite formation is closely allied to that of biotite which is also concentrated in the melanosomes

groundwater salinity as a control on development of eolian landscape an example from the white sands of new mexico abstract the white sands of southern new mexico form the largest field of gypsum dunes in the world the juxtaposition of several different types of dunes within the field has long been recognized and has generally been attributed to changes in the rate of sand transport across the dune field however steven fryberger fryberger s g 2003 geology of white sands national monument web page www2 nature nps gov geology parks whsa recently hypothesized however that a primary control is groundwater salinity the white sands dune field is underlain by saline waters fryberger proposed that parabolic dunes could form in topographic highs that accumulated a lens of fresher water derived from precipitation water with lower salinity allowed vegetation to grow and stabilized the sand except for the active noses of the parabolic dunes this study is a test of that hypothesis a study transect was established across the boundary between parabolic and barchan dunes groundwater and soil at six sites were sampled during december 2004 and march may and june of 2005 two sites were established in the parabolic field two in the barchans and two in the transition zone between them groundwater was found to be three times more saline in the barchan area conductivity and chloride decreased across the transition zone a gps topographic survey revealed two abrupt topographic steps one at the boundary between the barchans and the transition zone and a second at the edge of the parabolic field a second larger topographic step was found outside the study area defining a higher parabolic dune field these data indicate that fryberger s 2003 hypothesis is correct and groundwater is controlling the dune field morphology the dune field did not begin to form until the mid holocene 6500 years ago a compilation of dates shows that the parabolic dune field has probably been stable for the last 3500 years in contrast the barchan area has been recently deflated and exposed 2000 year old sediment near the surface

daily cycles in coastal dunes abstract daily cycles of summer sea breezes produce distinctive cyclic foreset deposits in dune sands of the texas and oregon coasts in both areas the winds are strong enough to transport sand only during part of the day reach a peak during the afternoon and vary little in direction during the period of sand transport cyclicity in the foreset deposits is made evident by variations in the type of sedimentary structure the texture and the heavy mineral content of the sand some of the cyclic deposits are made up entirely of one basic type of structure in which the character of the structure varies cyclically for example the angle of climb in a climbing wind ripple structure may vary cyclically other cyclic deposits are characterized by alternations of two or more structural types variations in the concentration of fine grained heavy minerals which account for the most striking cyclicity arise mainly because of segregation on wind rippled depositional surfaces where the ripples climb at low angles the coarsegrained light minerals which accumulate preferentially on ripple crests tend to be excluded from the local deposit daily cyclic deposits are thickest and best developed on small dunes and are least recognizable near the bases of large dunes

saltation threshold on earth mars and venus new formulations valid for wide ranges of particle diameter and density and gas density are presented for prediction of saltation threshold speed for small particles a low air density wind tunnel was used to extend the range of previous investigations and to separate the effects of reynolds number and interparticle forces of cohesion the new formulations are used to predict saltation threshold for atmospheric conditions on the surface of the earth mars and venus

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

p t t evidence for mechanisms of vertical tectonic motion in a contractional orogen north western us and canadian cordillera the 1800 km long coast mountains north cascades orogen of the canadian cordillera and north western us developed as a continental magmatic arc metamorphic rocks in the orogen contain widespread evidence for burial of supracrustal rocks to depths of c 40 km followed by nearly isothermal decompression to depths of 10 km near many shallowly emplaced mid cretaceous plutons low pressure contact metamorphic effects were overprinted by high pressure regional metamorphic minerals and textures as evidenced by kyanite staurolite pseudomorphs after andalusite in metapelitic rocks therefore near pluton rocks record the loading history of the orogen metapelitic rocks not associated with plutons only preserve evidence for high pressure conditions and or high temperature decompression as indicated for example by sillimanite and cordierite after kyanite and garnet respectively petrological evidence for burial and decompression is therefore recorded in different rocks various regions of the orogen differ in timing of metamorphism the overall shape of p t paths and the relative timing and regional extent of the high pressure event but most of these data and observations are consistent with thrusting and or pure shear thickening as primary loading mechanisms throughout the orogen as opposed to magma dominated loading this interpretation is further supported by comparison with thermal models which demonstrate that the p t paths are consistent with simultaneous thrusting and folding at a high initial geothermal gradient 35 40 c km 1 in much of the orogen a high geothermal gradient supports tectonic models invoking intra arc contraction and suggests that magmatism played an important role in regional temperature time paths this tectonic thermal history may be typical of other contractional orogens and illustrates the importance of large vertical displacement of crust in magmatic arcs

time scales of metamorphism deformation and crustal melting in a continental arc north cascades usa the record of metamorphism deformation and melting in the north cascades continental arc provides insights into the timing and mechanisms of extensional unroofing that followed crustal thickening the skagit gneiss north cascades is composed of variably deformed 90 45 ma tonalitic to granitic intrusive rocks these lithologies and metasedimentary rocks are migmatitic zircon and monazite from different textural varieties of migmatite from three outcrops along an east west transect were analyzed using the isotope dilution thermal ionization mass spectrometry id tims technique the data reveal two main migmatization pulses 1 68 63 ma and 2 53 47 ma in the westernmost locality leucosome zircon yields group 1 dates and mesosome sillimanite garnet biotite gneiss zircon is as young as 61 ma leucosome zircon in the easternmost outcrop yields only group 2 dates and biotite gneiss contains 47 ma zircon the third outcrop has leucosome dates from both groups 65 ma and 53 ma monazite from leucosomes and mesosomes likely records the timing of prograde metamorphism ca 69 ma and later synkinematic fluid mediated growth 49 46 ma these results indicate that partial melting occurred during 71 61 ma metamorphism and lasted through 46 ma deformation associated with exhumation the results and field observations suggest the presence of ductilely deforming crust from 68 to 46 ma the similarity between the youngest u pb dates in the migmatites and cooling and basin filling ages suggests a link between ductile and brittle processes over a range of structural levels in the arc from a zone of crustal melting flow and migmatite crystallization to earth9s surface

sediment transport part i bed load transport a method is presented which enables the computation of the bed load transport as the product of the saltation height the particle velocity and the bed load concentration the equations of motions for a solitary particle are solved numerically to determine the saltation height and particle velocity experiments with gravel particles transported as bed load are selected to calibrate the mathematical model using the lift coefficient as a free parameter the model is used to compute the saltation heights and lengths for a range of flow conditions the computational results are used to determine simple relationships for the saltation characteristics measured transport rates of the bed load are used to compute the sediment concentration in the bed load layer a simple expression specifying the bed load concentration as a function of the flow and sediment conditions is proposed a verification analysis using about 600 alternative data shows that about 77 of the predicted bed load transport rates are within 0 5 and 2 times the observed values

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

stratigraphic evolution of an aeolian erg margin system the permian cedar mesa sandstone se utah usa the permian cedar mesa sandstone of south east utah is a predominantly aeolian succession that exhibits a complex spatial variation in sedimentary architecture which in terms of palaeogeographic setting reflects a transition from a dry erg centre through a water table controlled aeolian dominated erg margin to an outer erg margin subject to periodic fluvial incursion the erg margin succession represents a wet aeolian system accumulation of which was controlled by progressive water table rise coupled with ongoing dune migration and associated changes in the supply and availability of sediment for aeolian transport variation in the level of the water table relative to the depositional surface determined the nature of interdune sedimentary processes and a range of dry damp and wet flooded interdune elements is recognized variations in the geometry of these units reflect the original morphology and the migratory behaviour of spatially isolated dry interdune hollows in the erg centre locally interconnected damp and or wet interdune ponds in the aeolian dominated erg margin and fully interconnected fluvially flooded interdune corridors in the outer erg margin relationships between aeolian dune and interdune units indicate that dry damp and wet interdune sedimentation occurred synchronously with aeolian bedform migration temporal variation in the rates of water table rise and bedform migration determined the angle of climb of the erg margin succession such that accumulation rates increased during periods of rapidly rising water table whereas sediment bypassing zero angle of climb occurred in the aftermath of flood events in response to periods of elevated but temporarily static water table during these periods in the outer erg margin the expansion of fluvially flooded interdunes in front of non climbing but migrating dunes resulted in the amalgamation of laterally adjacent interdunes and the generation of regionally extensive bypass flood supersurfaces a spectrum of genetic depositional models is envisaged that accounts for the complex spatial and temporal evolution of the cedar mesa erg margin succession

aeolian stratification and facies association in desert sediments arran basin permian scotland abstract permian aeolian sediments on the island of arran are divisible into dune including draa and interdune deposits both types display a distinctive and unusually wide variation in grain size the dominant features of the dune deposits are grainfall lamination sandflow lamination and inverse graded lamination associated with ripple form lamination and normal graded lamination the flat lying aeolian interdune deposits are characterised by granule and sand ripples horizontal lamination in coarse sand and granules plane bed lamination and inverse graded lamination associated structures include ripple form lamination and deflation lags three types of trace fossil associated with completely bioturbated horizons occur in some low angle dune and interdune deposits the aeolian facies interfinger with alluvial fan deposits giving rise to three recognizable facies belts marginal aeolian deposits are associated with fluvial conglomerates and are dominated by interdune deposits and occasionally very thin barchan deposits set height 3 37 cm intermediate aeolian deposits are characterized by interbedded crescentic dune small draa dune set height 5 cm 4 5 m and interdune deposits and rare fluvial and lake sediments basinal aeolian deposits are dominated by draa deposits dune set height 0 2 28 m associated with rare interdune sediments transverse dunes and draas were moved by north eastern palaeowinds towards the foot of the alluvial fans the aeolian sediments were deposited in a fault bounded desert basin

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

fluid inclusion geochemistry and ore genesis of the longmendian mo deposit in the east qinling orogen implication for migmatitic hydrothermal mo mineralization abstract the 1 85 ga longmendian mo deposit in east qinling is the oldest mo system recognized in china it is unique for three reasons 1 quartzofeldspathic orebodies are away from any intrusion or fault 2 it is closely associated with migmatitic rocks and 3 it has remarkably high re content in molybdenite 504 to 1660 ppm the origin of the deposit is poorly constrained in the longmendian deposit strong mo mineralization is always associated with hydrothermally altered migmatitic amphibolites to probe into ore genesis detailed fluid inclusion studies are carried out on both mineralized migmatitic amphibolites and ore barren rocks four compositional types of fluid inclusions are observed including co 2 ch 4 pc type co 2 h 2 o c type daughter mineral bearing s type and h 2 o nacl w type quartz in mesosome and melanosome of mineralized migmatitic amphibolites contains all of the four types of inclusions measurements of immiscible inclusion assemblages in the ores constrain the mo mineralization temperatures and pressures to be 225 390 c and 114 265 mpa respectively primary fluid inclusions in barren migmatitic gneisses are dominated by s type and minor of c and pc types which are identical to those in the leucosome of migmatitic amphibolites these inclusions yield lower homogenization temperatures than those in the mineralized mesosome and melanosome of migmatitic amphibolites suggesting that the ore causative injected melts should have higher temperatures and originated from depths such features of the ore forming fluids indicate that the longmendian deposit was a migmatitic hydrothermal system caused by high temperature melt injection this interpretation can also be supported by the observations below 1 tourmaline is abundant in melanosome but absent in mesosome 2 the consistent quartzofeldspathic composition of leucosome is independent of mesosome 3 leucosome in migmatitic amphibolite crosscuts each other 4 halite bearing fluid inclusions are prevalent in studied samples which is the feature of granitic rocks instead of the scenarios of in situ migmatites and 5 the trapping temperatures of fluid inclusions in mineralized migmatitic amphibolite are much lower than those required for partial melting but higher than those obtained from barren migmatitic rocks

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

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

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

origin of terrestrial gypsum dunes implications for martian gypsum rich dunes of olympia undae abstract the estancia white sands guadalupe and cuatrocienegas dune fields are among the largest known aeolian gypsum sand dune accumulations on earth and occupy closed drainage basins within the rio grande rift high sedimentation rates of lacustrine gypsum occur in topographic depressions within the closed basins the gypsum accumulations result from long term complex interaction between tectonism climate and a hydrologic cycle that involves geochemical recycling of sulfur from older sedimentary rocks flanking and underlying the basins gypsum precipitation in lacustrine environments is strongly controlled by local groundwater bedrock interaction the ranges of 34s values per mil vs vcdt for gypsum sand in the white sands dune field 12 1 to 13 9 guadalupe dune field 10 2 to 12 5 and cuatrocienegas dune field 14 6 to 15 9 indicate that the main sources of dissolved sulfate are evaporite strata of lower permian middle permian and cretaceous ages respectively a spatial increase of 34s values across the white sands dune field in the direction of prevailing winds matches well with a stratigraphically upward increase of 34s values recorded in lake otero an ancient lake in the basin strata this finding supports previous suggestions that white sands dune field evolved as a result of the step wise deflation of previously stored sediments in lake otero the modern gypsum dune fields are primarily wet eolian systems in which dune accumulation is controlled by a near surface groundwater table which promotes early cementation of the dune accumulations early cementation in the interdune surfaces and to a lesser degree on the dune surfaces reduces the amount of sand available for transport and slows rates of dune migration based on an average migration rate of 2 m year the time needed for a dune at white sands to migrate the downwind distance of the dune field is approximately 6500 years which matches well with other estimates of the initiation of the dune field at approximately 7000 years ago and indicates that white sands likely evolved as a wet aeolian system as on earth gypsum rich dune fields apparently are rare on mars gypsum has been identified only within the olympia undae dune field which encircles a portion of the martian north polar residual ice cap analogous to terrestrial gypsum dunes the gypsum within the olympia undae gypsum rich dunes might have originated from transport and deposition via aeolian processes in this model gypsum rich source sediment could have been formed by confined groundwater or surface water activity and later transported by the wind any subsequent gypsum precipitation in the source area could have been initiated by episodic melting of the martian polar ice cap it is likely that the gypsum was deposited in the olympia undae dunes after the main deflation events that led to formation of the siliciclastic components of the dune field