trace and minor elements in galena a reconnaissance la icp ms study minor and trace elements can substitute into the crystal lattice of galena at various concentrations in situ la icp ms analysis and trace element mapping of a range of galena specimens from different deposit types are used to obtain minor trace element data aimed at achieving insight into factors that control minor trace element partitioning the previously recognized coupled substitution ag bi sb 3 2pb2 is confirmed however the poorer correlation between ag and bi sb when the latter elements are present at high concentrations 2000 ppm suggests that site vacancies may come into play 2 bi sb 3 3pb2 galena is the primary host of tl in all mapped mineral assemblages along with cu tl is likely incorporated into galena via the coupled substitution ag cu tl bi sb 3 2pb2 tin can reach significant concentrations in galena 500 ppm cd and minor hg can be incorporated into galena the simple isovalent substitution cd hg 2 pb2 is inferred this paper shows for the first time oscillatory and sector compositional zoning of minor trace elements ag sb bi se te tl in galena from two epithermal ores zoning is attributed to slow crystal growth into open spaces within the vein at relatively low temperatures the present data show that galena can host a broader range of elements than previously recognized for many measured elements the data sets generated display predictable partitioning patterns between galena and coexisting minerals which may be dependent on temperature or other factors trace element concentrations in galena and their grain scale distributions may also have potential in the identification of spatial and or temporal trends within individual metallogenic belts and as markers of ore formation processes in deposits that have undergone superimposed metamorphism and deformation galena trace element geochemistry may also display potential to be used as a trace minor element vector approach in mineral exploration notably for recognition of proximal to distal trends within a given ore system

interdune areas of the back island dune field north padre island texas abstract the small young about 100 yrs back island dune field on north padre island south texas consists of fairly persistant oblique dunes up to 6 m high with well developed interdune areas that grade northwestward to small ephemeral transverse and barchan dunes with interconnected interdune areas thence sheet sand areas the subhumid climate is marked by rain associated with frontal systems and tropical storms winds are seasonally bimodal prevailing southeasterly are punctuated by northerly and northwesterly winds with the passage of frontal systems in winter the entire dune field and individual oblique dunes show a net migration of about 15 m yr 1 to the northwest the dunes however are on a seasonally reversing track changing their slipface direction and migration direction with frontal systems one year of monitoring shows sand transport in the dune interdune system to be complex and cyclic during the wind reversals of winter dunes are very ineffecfive sand traps owing to loss of flow separation and much sand is lost to the interdune areas interdune areas store sand during these wet winter months as a result of the wind reversals and higher moisture content during the summer the interdune areas deflate and the dunes build in size the overall dune field deposit appears to consist of three laterally contiguous zones from southeast to northwest 1 continuous climbing oblique dune and interdune deposits 2 discontinuous lenses of dune sand in overall interdune layers and 3 a chaotic mixture of dune and horizontal deposits of the sheet sand areas one year s mapping and trenching documents that interdune sedimentary structures are extremely variable laterally and vertically reflecting specific microenvironments within the interdune flat wet surface features consist of current and wave ripples channel fill miniature deltas wrinkle marks mini ripples rills algae and sand volcanoes abundant adhesion structures rain impacted ripples brecciated surfaces and microtopography reflect damp surface deposits dry surface features are predominately wind ripples others include small isolated barchan and shadow dunes organic debris lag surfaces deflation scours beetle bioturbation plant root structures associated with shadow dunes and grainfall from the adjacent dunes interdune deposits account for about 40 of the total dune field deposits which seems reasonable compared to some ancient examples by virtue of occupying a relative basin interdune deposits are selectively preserved compared to dune deposits in general interdune sedimentation is enhanced by non eolian depositional mechanisms a high water table early evaporatic cements and a variable wind regime the actual thicknesses of individual dune and interdune deposits are less on padre than ancient examples reflecting the relative scale of the bedforms in many respects sequences of sedimentary structure in padre island interdune deposits are typical of ancient coastal interdune strata but some marked departures occur adhesion structures relatively rare in some ancient examples but abundant within padre interdune deposits seem favored by the small size of dune and interdune area the climate and a variable wind regime penecontemporaneous deformation absent in padre interdune deposits but pronounced in some ancient examples probably reflects dune size and the nature of the deposits wavy laminae in ancient interdune deposits probably result from many causes but seem best represented by modern examples of evaporitic algal bacterial formed structures

basin tectonics and erg response abstract the alignment and distribution of facies in the major erg eolian sand sea deposits in the permian and jurassic systems on the colorado plateau show remarkable coincidence with modern tectonic elements this indicates that subtle tectonic elements precursors to the present ones were coeval with ancient eolian deposition and controlled erg geometry and facies distribution no other known controls or processes were capable of producing the observed patterns facies and architectural analysis of the permian and jurassic erg deposits define several major trends across the colorado plateau the high plateaus trend parallel to and slightly east of the wasatch line is defined by a zone of thinning and facies change of several erg deposits the zuni trend lies along the zuni lineament and is coincident with facies changes or erg margins of several units the colorado trend lies along the course of the modern colorado river and a number of erg margins and several facies changes coincide with it most erg centers as defined by facies and thickness lay within paleobasins that include the circle cliffs trough holbrook black mesa and blanding basins and southern cordilleran miogeocline most erg deposits thin or pinch out across paleoarches which include the sedona monument emery and defiance arches although the exact relation between the paleostructures and facies distribution is uncertain it appears likely that some of the structural elements defined areas of greater and less sand accumulation areas of greater sand accumulation would be expected to be dominated by facies typical of erg centers and areas of lesser sand accumulation would be expected to be dominated by facies typical of the erg margin and eolian sand sheets the structures may have also formed straight margins in aqueous environments such as rivers and marine coasts straight vertically stacked facies changes are commonly associated with erg margins in the stratigraphic record early post depositional uplift of some arches may have defined straight thin edges of some eolian sequences the knowledge that subtle intrabasinal tectonic elements had an effect on eolian deposition can be used to reconstruct basin history therefore the mapping of eolian geometry and facies patterns can provide powerful information concerning the tectonic evolution of a sedimentary basin

formation of scalloped cross bedding without unsteady flows abstract scalloped cross bedding compound cross bedding with internal bounding surfaces that cyclically scoop into the previously deposited foresets and into the sediment below the set is a common and distinctive structure in eolian fluvial tidal and nearshore marine sands scalloped cross bedding in shallow marine deposits previously has been interpreted to be produced by cyclic flows such as neap spring tidal flows which are known to cause cyclic fluctuations in the depth of scour in the troughs of migrating bedforms but scalloped cross bedding also originates by a process that does not require fluctuating flow migration of small bedforms across the lee slopes or along the troughs of larger bedforms intersections of the troughs of the two sets of bedforms form topographicall low scour pits and cyclic passage of these scour pits through the outcrop plane the plane that later becomes an outcrop surface causes the lower set boundary to rise and fall scalloped cross bedding formed by fluctuating flow superficially resembles that formed by superimposed or intersecting bedforms but as illustrated in three dimensional computer plots the two kinds of structures commonly can be distinguished by directional properties of the bedding scallops deposited by alongslope migrating superimposed bedforms have cross bed and bounding surface dip patterns that lack bilateral symmetry and have cross bed dips that are asymmetrically distributed relative to bounding surface dips scallops with dip patterns that are bilaterally symmetrical and with cross bed dips that are symmetrically distributed relative to the bounding surface dips can be produced either by fluctuating flow or by downslope or upslope migration of superimposed bedforms an example of nearshore marine scalloped cross bedding of pleistocene age was examined in detail in a coastal terrace of monterey bay california the three dimensional structure and directional properties of the bedding suggest that the deposit was produced by a series of small bedforms migrating offshore down a rip channel that was bounded on one side by a migrating oblique bar

hoanib river flood deposits of namib desert interdunes asanalogues for thin permeability barrier mudstone layers inaeolianite reservoirs abstract the ephemeral braided hoanib river of nw namibia ows for a few days ayear and only high discharges enable the river to pass through interdunaldepressions within the northern namib desert dune eld to the atlantic thedune eld comprises mainly large transverse dunes resulting frompredominant ssw winds river ood deposits between aeolian dunes areanalogous to mudstone layers conformably interbedded with ancientaeolianite dune foresets deep oods pond laterally to considerable depths metres to 10 m in adjacent interdunes depositing mud layers 1 50 cm toconsiderable heights on avalanche and stoss faces of bounding dunes fairlypassive ooding only disturbs aeolian strati cation minimally floodwaterclay in ltrates and settles as an impermeable seal with a ood pond on top perched above regional groundwater flood ponds evaporate slowly for longperiods 3 years early emergence desiccates higher parts of a mud layer subsequent oods can re ll a predecessor pond bene ting from the existingimpervious seal potential preservation of such mud layers is lower on thestoss face but high on the avalanche face after burial by subsequent dunereactivation and migration the leeward right hoanib bank a dune stoss face is river and wind eroded to exhume fossil interdune pond mud layers of anearlier hoanib channel the highly inclined layers are interbedded with duneavalanche foresets and represent the edges of two successive fossil pondsexposed in plan ancient ood pond mudstones occur in the permian triassichydrocarbon reservoir the sherwood sandstone group of the cheshire basin kinnerton formation and irish sea basin and were previously usederroneously to argue against the aeolian origin of cross bed sets hoanibstudies show that primary river interaction with a dune eld might preserveonly localized erosional omission surfaces in ancient aeolianites with littlesandy barform preservation prone to aeolian reworking around the main uvial channel locus however ood pond mudstone layers should form apredictable halo within which uid permeability will decrease keywords aeolian dunes aeolianite reservoir ash ood interdune namibdesert namibia permeability introductionhigh discharge oods of the hoanib river occa sionally break through the skeleton coast dune eld which forms part of the extensive namibdesert that parallels the western margin of thesouthern african subcontinent this arid regionis 140 km wide and over 2000 km long fromsouth western angola to the northern capeprovince south africa the northern namib dune eld termed the skeleton coast dune eld bywilson 1973 lancaster 1982 and ward sedimentology 2002 49 719 736

tropical westerlies over pangaean sand seas cross equatorial westerly winds are key features of tropical circulation in monsoonal regions although prominent in numerical climate models of pangaea the supercontinent straddling earth s equator late palaeozoic to early mesozoic such flow has not been confirmed previously by migration directions of ancient dunes wind blown sandstones that span 100 million years of earth history are widely exposed in south western usa if recent palaeomagnetic data from the colorado plateau are used to correct mesozoic palaeogeographic maps the plateau is placed about 10 further south than previously assumed and the prevailing north westerly surface winds recorded by dune deposited sandstones are explicable as cross equatorial westerlies the hallmark of modern monsoon circulation permian to early jurassic dunes were driven by north westerlies produced by a steep pressure gradient spanning the supercontinent during december january february although winds are light in most modern near equatorial settings the east african jet accounts for more than half the cross equatorial flow in june july august the thicknesses of annual depositional cycles within the navajo sandstone indicate that the near equatorial north westerly winds that drove these particular dunes were stronger than the modern east african jet the early jurassic dunes that deposited the thick cycles were positioned west of the dominant southern hemisphere thermal low and against highlands to the west a setting very similar to the east african jet the mountains along the western coast of pangaea not only enhanced wind strength but also cast a rain shadow that allowed active dunes to extend very close to the palaeoequator

tidally flooded back barrier dunefield guerrero negro area baja california mexico a 45 km long barrier island exists west of the town of guerrero negro mexico along the western coast of the baja california peninsula about 720 km south of san diego california this barrier has developed in a mesotidal arid climate regime characterized by steady strong onshore winds from the nw the barrier island w of guerrero negro has prograded seaward about 1 6 km in the last 1800 years while an aeolian dunefield fed by sand blown from beaches has advanced inland up to 13 km landward progradation of the dune system from the barrier has occurred during relative rise in sea level thus aeolian sediments exist at or below the water table over a wide area the progradation of dunes across marshes tidal flats and tidal channels as well as the repeated submergence of interdune areas by tidal waters has created a complex suite of mixed aeolian and subaqueous sediments in the back barrier the complexity of the suites of aeolian sedimentary structures together with the inclusion of subaqueously formed structures such as current and oscillation ripples would make recognition of the aeolian origin of much of the sediments difficult in ancient rocks in addition to the scientific importance of recognizing the aeolian deposits the sedimentation model represented by the guerrero negro barrier has applications in petroleum exploration and development currently most preservational models for barrier islands attach little volumetric importance to aeolian deposits this modern example suggests that volumetrically significant aeolian deposits can be preserved behind a barrier particularly in an arid climate regime if preserved and charged with oil the resulting productive sandstone could have an extremely irregular landward edge comprised in part of onshore prograded aeolian dune sandstone with excellent reservoir characteristics as with current barrier models the reservoir would be sealed landward and above by lagoonal mudstone and silt evaporites or evaporitic sandy sabkha deposits high organic productivity occurs in lagoons immediately adjacent to the dunefields of guerrero negro suggesting that organic rich source rock may exist near aeolian sandstone in ancient settings similar to guerrero negro

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

geometry and origin of large scale deformation structures in some ancient wind blown sand deposits large scale deformational features that disrupt the cross bedded strata of some ancient wind blown dune sand deposits jurassic aztec and navajo sandstones u s a can betraced laterally for at least tens of metres information from four exposures leads to an idealized deformation style characterized by 1 a head portion marked by collapse features 2 a middle portion marked by a thrust ramp or a large recumbent fold and 3 a toe portion marked by a planar shear zone with small recumbent folds drag folds and possibly small horizontal shear faults an hypothesis involving earthquake induced liquefaction and collapse of the dunes is proposed sands liquefied during earthquakes cannot support an unequal surface load resulting in the collapse of surface dunes above the liquefied substrate compensatory lateral squeezing of liquefied sand and buried strata produces the shear zone in the toe portion engineering studies suggest that buried strata underlying interdunal lows where over burden stress is least will be most susceptible to liquefaction other considerations suggest that it will be the steeper lee sides of advancing surface dunes that collapse squeezing liquefied sand forward or downwind i e away from the lee slope of surface dunes if the liquefied condition persists load structures can be formed when unliquefied layers founder into the liquefied substrate

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

reconstructing random topography from preserved stratification one of the fundamental problems of interpreting the sedimentary record is reconstructing the original heights of palaeotopographical features such as bedforms or river channels this requires an understanding of the relationship between topography and set thickness but at present an exact theory exists only for periodic topography of uniform height the applicability of this simple theory is severely limited by the random variability characteristic of many sedimentary systems in this paper we develop an exact theory for the probability density function pdf of sets generated from topography of random height we focus on the limiting case of zero net deposition in order to provide a lower bound for the set thickness and derive an analytical set thickness pdf that is determined by one parameter of the pdf for topographical height this parameter measures the breadth of the tail of the topographical pdf the mean set thickness is 0 8225 for bedforms and 1 645 for river channels if the topographical height is gamma distributed the preservation ratio defined as mean thickness of preserved sets mean topographical height is 0 8225 r2 for bedforms and 1 645 r2 for river channels where r is the coefficient of variation standard deviation mean of the generating topography in a comparison with data from laboratory current ripples our analytical predictions compare well with observations of both mean set thickness and thickness distribution the preservation ratio for the ripples is about 0 5 whilst measured dune heights give a preservation ratio of about 0 12 depth data from two modern braided streams yield preservation ratios ranging from 0 4 to 0 75 as more data on the distributions of topographical height in modern environments become available calculations such as these should help provide reliable error bounds for quantitative topographical reconstruction

fluvial aeolian interactions part i modern systems two modern fluvial aeolian depositional systems great sand dunes national monument colorado and the mojave river wash california are remarkably similar in spite of different climates sizes fluvial sediment textures and relative directions of aeolian and fluvial transport dune growth and migration and deflation of blowouts create 8 10 m of local relief in unflooded aeolian landscapes there are six prominent fluvial aeolian interactions 1 fluvial flow extends into the aeolian system until it is dammed by aeolian landforms 2 interdune areas overbank interdunes upstream of aeolian dams and alongside channels are flooded 3 water erodes dunes alongside channels and interdunes 4 flood waters deposit sediment in interdune areas 5 fluvially derived groundwater floods interdunes interdune playas 6 wind erodes fluvial sediment and redeposits it in the aeolian system unique and characteristic sediments are deposited in overbank interdunes and in interdune playas reflecting alternate fluvial and aeolian processes and rapidly changing flow and salinity conditions these fluvial aeolian interdune deposits are characterized by irregular concave up bases and flat upper surfaces containing mudcracks or evaporite cement characteristic low relief surfaces form in aeolian systems as an effect of flooding fluvial deposits are resistant to aeolian deflation aeolian sand is preserved when flood sediments are deposited around the bases of the dunes thus repetitive fluvial and aeolian aggradation tends to be stepwise as interdune floors are suddenly raised during floods the effects of flooding should be easy to recognize in ancient aeolianites even beyond the area covered with overbank muds

stratigraphic evolution and preservation of aeolian dune and damp wet interdune strata an example from the triassic helsby sandstone formation cheshire basin uk new and previously published models of wet aeolian system evolution form a spectrum of types that may be explained in terms of aeolian dune dynamics rate of water table rise and or periodicity of interdune flooding this is illustrated with an example from the mid triassic anisian helsby sandstone formation cheshire uk lenses of damp and wet interdune strata exhibit an intertonguing transitional relationship with the toe sets of overlying aeolian dune units this signifies dune migration that was contemporaneous with water table controlled accumulation in adjacent interdunes downwind changes in the geometry and facies of the interdune units indicate periodic expansion and contraction of the interdunes in response to changes in the elevation of the groundwater table and episodic flooding during which accumulation of dune strata continued relatively uninterrupted this contrasts with other models for accumulation in wet aeolian systems where interdune flooding is associated with a cessation in aeolian bedform climbing and the formation of a bypass or erosional supersurface architectural panels document the detailed stratigraphy in orientations both parallel and perpendicular to aeolian transport direction enabling a quantitative three dimensional reconstruction of genetically related aeolian dune and interdune elements sets of aeolian dune strata are composed of grainflow and translatent wind ripple strata and are divided by a hierarchy of bounding surfaces originating from oblique migration of superimposed dunes over slipfaceless sinuous crested parent bedforms together with lee slope reactivation under non equilibrium flow conditions silty mudstone and sandstone interdune units are characterized by wind ripple wavy and subaqueous wave ripple laminae desiccation cracks mud flakes raindrop imprints load casts flutes intraformational rip up clasts and vertebrate and invertebrate footprint impressions and trackways these units result from accumulation on a substrate that varied from dry through damp to wet surface conditions interdune ponds were flooded by either fluvial incursions or rises in groundwater table and were periodically subject to gradual desiccation and reflooding red silty mudstone beds of subaqueous origin pass laterally into horizontally laminated wind ripple beds indicating a progressive transition from wet through damp to dry surface conditions within a single interdune

tilt prior to explosions and the effect of topography on ultra long period seismic records at fuego volcano guatemala 1 ground tilt is measured from broadband seismic records prior to frequent explosions at fuego volcano guatemala we are able to resolve tilt beginning 20 30 minutes prior to explosions followed by a rapid reversal in deformation coincident with explosion onsets the tilt amplitude and polarity recorded on the horizontal channels vary from station to station such that the steep and unusual topography of the upper cone of fuego appears to affect the ultra long period signals we account for the effect of topography and attempt to constrain the tilt source depth and geometry through finite difference modeling the results indicate a shallow spherical pressure source and that topography must be considered when attempting to model tilt sources at volcanoes with steep topography the tilt signals are interpreted as pressurization of the shallow conduit beneath a crystallized plug followed by elastic deflation concurrent with explosive pressure release

periodic accumulation and destruction of aeolian erg sequences in the permian cedar mesa sandstone white canyon southern utah usa the permian cedar mesa sandstone represents the product of at least 12 separate aeolian erg sequences each bounded by regionally extensive deflationary supersurfaces facies analysis of strata in the white canyon area of southern utah indicates that the preserved sequences represent erg centre accumulations of mostly dry though occasionally water table influenced aeolian systems each sequence records a systematic sedimentary evolution enabling phases of aeolian sand sea construction accumulation deflation and destruction to be discerned and related to a series of underlying controls sand sea construction is signalled by a transition from damp sandsheet ephemeral lake and palaeosol deposition through a phase of dry sandsheet deposition to the development of thin chaotically arranged aeolian dune sets the onset of the main phase of sand sea accumulation is reflected by an upward transition to larger scale ordered sets which represent the preserved product of climbing trains of sinuous crested transverse dunes with original downwind wavelengths of 300 400 m regularly spaced reactivation surfaces indicate periodic shifts in wind direction which probably occurred seasonally compound co sets of cross strata record the oblique migration of superimposed slipfaced dunes over larger slipfaceless draa each aeolian sequence is capped by a regionally extensive supersurface characterized by abundant calcified rhizoliths and bioturbation and which represents the end product of a widespread deflation episode whereby the accumulation surface was lowered close to the level of the water table as the sand sea was progressively cannibalized by winds that were undersaturated with respect to their potential carrying capacity aeolian sequence generation is considered to be directly attributable to cyclical changes in climate and related changes in sea level of probable glacio eustatic origin that characterize many permo carboniferous age successions sand sea construction and accumulation occurred during phases of increased aridity and lowered sea level the main sand supply being former shallow marine shelf sediments that lay to the north west sand sea deflation and destruction would have commenced at or shortly after the time of maximum aridity as the available sand supply became exhausted restricted episodes of non aeolian accumulation would have occurred during humid interglacial phases accumulation and preservation being enabled by slow rises in the relative water table subsidence analysis within the paradox basin together with comparisons to other similar age successions suggests that the climatic cycles responsible for generating the cedar mesa erg sequences could be the product of 413 000 years so called long eccentricity cycles by contrast annual advance cycles within the aeolian dune sets indicate that the sequences themselves could have accumulated in just a few hundred years and therefore imply that the vast majority of time represented by the cedar mesa succession was reserved for supersurface development

relating eolian bounding surface geometries to the bed forms that generated them etjo formation cretaceous namibia the cretaceous etjo formation is a 200 m thick eolian and fluvial sandstone succession exposed in the huab basin of northwestern namibia eolian sedimentation was terminated abruptly by the emplacement of flood basalts of the etendeka igneous province across much of the basin at 132 1 ma the lavas drowned the dunes and in doing so preserved bed forms with heights and wavelengths of up to 100 m and 1 3 km respectively subsequent erosion has resulted in the exposure of these bed forms and provides a rare opportunity to accurately reconstruct the three dimensional geometry of an ancient eolian system and to relate bed form morphology to bounding surface geometries some bed forms lacked a single active slipface but instead were characterized by a dome form over which smaller transverse dunes migrated other bed forms were characterized by an active slipface and a stoss slope over which smaller superimposed bed forms migrated one bed form records a transitional evolution between these two end member types preserved bed sets exhibit a positive but stoss erosional angle of downwind bed form climb of 1 bed forms of the etjo formation provide an opportunity to directly relate bed set thickness to original bed form height via the angle of climb

dune and dune field development on padre island texas with implications for interdune deposition and water table controlled accumulation abstract portions of the back island dune fields on padre island texas undergo seasonal destructive and constructive phases in which they are reduced to a nearly planar surface during the winter and then reform during the spring and summer initial depositional sites for dry sand that we observed were associated with roughness elements that caused a lowering of the transport capacity of the flow bedforms develop through a series of morphologic and dynamic stages 1 irregular patches of dry sand 2 wind ripple protodunes with leeward flow expansion 3 grainfall protodunes with a range from leeward flow expansion to flow separation 4 barchan dunes with grainflow and 5 crescentic ridges that characterize the mature field the survival and further development of protodunes and dunes is f vored by increased size a more advanced stage of development unidirectional and moderate wind and a plentiful sand supply these factors can be viewed as the result of the interrelationship of aspects of eolian transport the nature of leeward secondary airflow patterns and sand supply dune field ordering occurs by repeated merging splitting cannibalization and lateral linking of dunes on padre island and other areas of dunes that we studied flow conditions across portions of interdune flats favor the transport of dry loose sand from interdune flats onto the dunes so that dunes grow at the expense of interdune flats dry surface deposits are therefore unlikely to accumulate on interdune flats but rather to characterize interdune depressions that may represent the minimum d stance to which an interdune flat can be closed otherwise accumulations on interdune flats most commonly should be formed on wet or damp surfaces or occur associated with some mechanism that protects the deposits from deflation padre island dune fields represent a type of eolian system where accumulations of dunes and interdune flats occur during periods of a water table rise with bypassing and deflation occurring as the water table becomes static or falls respectively

subaerial debris flow deposition in the upper paleozoic cutler formation western colorado abstract outcrops of the lower part of the cutler formation near gateway colorado contain proximal alluvial fan deposits of two principal lithofacies the diamictite lithofacies d consists of a variety of sharply bounded massive muddy beds which are interpreted as deposits of subaerial debris flows the gravelly sandstone lithofacies s consists of amalgamated beds of laminated and cross bedded conglomeratic sandstones which are interpreted as water deposited sediments clasts in s are rarely larger than 0 5 m in maximum diameter but clasts in d are commonly much larger up to 8 m both normal and inverse grading are visible in d but grading is generally not well developed in either lithofacies imbrication of clasts although more pronounced and more consistent in s is fairly commo in both lithofacies four subfacies of diamictite are distinguishable on the basis of clast packing and grading massive matrix supported dram graded matrix supported dmg inversely graded clast supported dci and massive clast supported dcm given the hydrodynamic and rheologic properties of debris flows as reported by previous workers four debris flow regimes are postulated on the basis of varying water content and clast interaction subfacies dram and drag typify deposits of flows having viscous clast interactions subfacies dci represents deposits of flows having inertial or colliding clast interactions subfacies dram comprises deposits of flows with demonstrably high yield strengths subfacies dcm represents deposits of flows in which yield strength was insufficient to support most clasts this classification specifically recognizes turbulence selective deposition and bedload transport of clasts as essential characteristics of certain dilute debris flows it also accommodates as debris flow deposits some beds which have features usually associated with aqueous deposition including clast frameworks channelization imbrication and normal grading

episodic deposition and preservation of eolian sands a late paleozoic example from southeastern utah the upper pennsylvanian and lower permian stratigraphic sequence in canyonlands national park contains about 40 flat topped eolian sand bodies rhizoliths burrows and traces of former evaporites abound at the tops of these tabular genetic units upper surfaces of units were created by eolian deflation to the level of the groundwater table analysis of cross stratification reveals that sands accumulated via migration and climb of relatively small dunes brief episodes of sand accumulation were triggered by regression and were terminated when upwind sand supplies were depleted upper surfaces of genetic units are diastems and represent much more geologic time than the rocks themselves

braided rivers within an arid alluvial plain example from the lower triassic western german basin recognition criteria and expression of stratigraphic cycles based on a detailed sedimentological analysis of lower triassic continental deposits in the western germanic sag basin i e the eastern part of the present day paris basin the conglomerat basal gres vosgien and conglomerat principal formations three main depositional environments were identified i braided rivers in an arid alluvial plain with some preserved aeolian dunes and very few floodplain deposits ii marginal erg i e braided rivers aeolian dunes and aeolian sand sheets and iii playa lake an ephemeral lake environment with fluvial and aeolian sediments most of the time aeolian deposits in arid environments that are dominated by fluvial systems are poorly preserved and particular attention should be paid to any sedimentological marker of aridity such as wind worn pebbles ventifacts sand drift surfaces and aeolian sand sheets in such arid continental environments stratigraphic surfaces of allocyclic origin correspond to bounding surfaces of regional extension elementary stratigraphic cycles i e the genetic units have been identified for the three main continental environments the fluvial type fluvial aeolian type and fluvial playa lake type at the time scale of tens to hundreds of thousands of years these high frequency cycles of climatic origin are controlled either by the groundwater level in the basin or by the fluvial siliciclastic sediment input supplied from the highland lower triassic deposits from the germanic basin are preserved mostly in endoreic basins the central part of the basin is arid but the rivers are supplied with water by precipitation falling on the remnants of the hercynian variscan appalachian mountains consequently a detailed study of alluvial plain facies provides indications of local climatic conditions in the place of deposition whereas fluvial systems only reflect climatic conditions of the upstream erosional catchments

fluvial aeolian interactions part ii ancient systems an understanding of fluvial aeolian deposition derived from modern case examples in a previous study is applied to the permian cutler formation and cedar mesa sandstone on the colorado plateau these formations supply an excellent three dimensional exposure of intertonguing fluvial and aeolian strata four distinct facies associations form the bulk of the cutler formation and cedar mesa sandstone 1 aeolian dune deposits 2 wet interdune deposits 3 fluvial channel deposits and 4 overbank interdune deposits in addition two distinctive types of erosion surfaces are found within the cutler formation and cedar mesa sandstone pebble to granule rich erosion surfaces aeolian deflation surfaces and flood surfaces fluvial and aeolian intertonguing result in extensive tabular sheets of aeolian sandstone separated by flood surfaces and overbank interdune deposits fluvial channels are associated with the deposits overlying flood surfaces and are incised into the underlying aeolian sandstones overbank interdune deposits and wet interdune deposits cover flood surfaces and intertongue with overlying aeolian sandstones the primary characteristics of ancient fluvial aeolian deposition are overbank interdune deposits and pronounced extensive erosion surfaces flood surfaces which are parallel to underlying fluvial sandstones and thus trend parallel to the palaeoslope and palaeohydrological gradient

aeolian dune and wet interdune sedimentation in the middle proterozoic dala sandstone sweden abstract the middle proterozoic dala sandstone covers an extensive area in west central sweden and the adjacent parts of norway but it is poorly exposed however a series of quarries in the lower part of the sandstone provides good exposures that allow an interpretation of the environment of deposition in this part of the unit only two facies have been distinguished facies a an aeolian dune facies and facies b a wet interdune facies facies a consists of large scale cross stratified sandstone in which cross stratified sets are up to almost 4 m thick while the maximum foreset dip is 25 sandflow and subcritically climbing translatent strata have been recognized in this facies but the identification of grainfall strata is uncertain penecontemporaneous folding and brecciation of cross strata has taken place locally facies b which was deposited simultaneously with facies a consists of sandstone with thin mud laminae an irregular wavy structure and numerous wave rippled and mud cracked surfaces the increasing proportion of facies b relative to facies a passing up the stratigraphic section may herald an approaching coastline lack of recognition rather than preservational bias is thought to be the reason for the apparent scarcity of aeolianites in the precambrian

interdune pond carbonates weber sandstone pennsylvanian permian northern utah and colorado abstract the eolian pennsylvanian permian weber sandstone predominantly a 300 m thick cross bedded quartz arenite contains carbonate lenses deposited in interdune ponds the carbonates are thin 60 cm laterally impersistent 100 1 000 m and consist chiefly of unfossiliferous fenestral dolomitic mudstone an interdune pond origin is supported by a lenticular three dimensional geometry b exclusive occurrence along first order bounding surfaces c lateral intertonguing with dry and damp interdune sandstone facies d lack of marine fossils and e megascopic features indicative of extremely shallow to emergent hypersaline to brackish conditions e g desiccation and sheet cracks fenestral fabrics and or burrow root tubes pseudomorphs after anhydrite nodules algal mat or str matolitic wrinkled laminations weber interdune ponds were probably fed by spring seeps and ground waters derived from deeper aquifers analogous in some respects to present day oasis ponds in the saharan erg of north africa interdune pond carbonates represent a relatively uncommon end member in a broad spectrum of interdune facies

challenging the sheetflood myth the role of water table controlled sabkha deposits in redefining the depositional model for the ormskirk sandstone formation lower triassic east irish sea basin

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

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

aeolian system sediment state theory and mojave desert kelso dune field example the sediment state of aeolian dune fields and sand seas at a basinal scale is defined by the separate components of sediment supply sediment availability and the transport capacity of the wind the sediment supply for aeolian systems is the sediment that contemporaneously or at some later point serves as the source material for the aeolian system numerous factors impact the susceptibility of grains on a surface to transport but these are cumulatively manifested by the actual transport rate which serves as a proxy for sediment availability transport capacity is the potential sediment transport rate of the wind because the three aspects of sediment state can be given as a volumetric rate they are directly comparable plotted simultaneously against time the generated curves define nine possible classes of sediment state sediment supply that is stored occurs because it is transport or availability limited or generated at a rate greater than the potential or actual transport rates respectively contemporaneous or lagged influx to an aeolian system may be limited by sediment availability but cannot exceed the transport capacity of the wind for the kelso dune field in the mojave desert of california a variety of stratigraphic and geomorphic evidence is used to approximate the sediment state of the system the sediment supply was generated during the latest pleistocene and earliest holocene during humid periods of enhanced discharge by the mojave river to form the lake mojave fan delta or terminal fan and has been calculated over time from the sedimentation rate and the frequency of floods estimation of transport capacity over time was based upon modern wind data with an allowance for greater winds during the pleistocene based upon climatic models sediment availability was approximated by calculation of a modern dune mobility index with variation over time based upon climatic inferences while quantifying the kelso or any natural system is subject to numerous uncertainties the sediment state approach reflects the temporal and spatial disjointed nature of accumulations at kelso as well as illuminating questions for future research

pennsylvanian to jurassic eolian transportation systems in the western united states peterson f 1988 pennsylvanian to jurassic eolian transportation systems in the western united states in g kocurek editor late paleozoic and mesozoic eolian deposits of the western interior of the united states sediment geol 56 207 260 the direction of sediment transport in eolian sandstones of permsylvanian to jurassic age was interpreted from crossbedding resultants vector means obtained from studies of eolian rocks in the western u s supplemented by data from the few eolian units of eastern north america these were compiled from the published or unpublished theses hterature from unpublished fidd data contributed by colleagues or from measurements made for this study in addition new paleogeographic maps were compiled to evaluate the influence of geographic features on the atmospheric circulation patterns that are inferred from the crossbedding studies regionally the crossbedding indicates northeasterly northerly or northwesterly winds present coordinates from pennsylvanian through most of middle jurassic time a rather abrupt change in wind directions occurred in late middle jurassic time late part of the cauovian age when westerly wind patterns developed by the late jurassic the winds shifted to southwesterly calculations of the consistency factor vector mean strength made from region wide analyses of the resultants indicate fairly unidirectional winds from the permsylvanian through the early jurassic middle jurassic circulation was more varied judging from crossbedding studies in the lower part of the entrada sandstone crossbedding in upper jurassic eolian rocks of wyoming and south dakota yielded a random pattern but upper jurassic rocks farther south on the colorado plateau and adjoining areas show a return to a fairly unidirectional pattern comparing the resultants with their reconstructed paleogeographic setting shows surprisingly little influence of major geographic features on overall circulation patterns however the greatest amount of local variation occurred at or near highly indented shorelines where the temperature contrast between land and water produces local wind currents that may vary appreciably from regional circulation patterns although they do not cause noticeable horizontal deflections in wind patterns small and low topographic highs appear to be able to promote the development of a dune field if a source of sand is available and if streams do not enter the growing dune field

conditions favourable for the formation of warm climate aeolian sand sheets aeolian sand sheets are areas of aeolian sand where dunes with slipfaces are generally absent sand sheets are ubiquitous to modern warm climate sand seas generally occurring marginal to dune fields although they may exist within the interior of a sand sea or independent of a dune field sand sheet deposits are recognized in ancient aeolian sequences where they may account for significant accumulations of low angle aeolian stratification we suggest that the occurrence of sand sheets instead of dunes indicates that conditions are outside the range within which dunes form or that one or more factors interfere with dune development while also favouring the accumulation of sand sheets a study of six modern sand sheets in north america located at great sand dunes gran desierto dumont algodones padre island and colorado river delta indicates that the factors favourable for sand sheet development are 1 a high water table 2 surface cementation or binding 3 periodic flooding 4 a significant coarse grained sediment population and 5 vegetation these factors are reflected in the nature of stratification and the accessory features of sand sheet accumulations within the areas of modern sand sheets as well as in their ancient counterparts in the triassic dolores and pennsylvanian permian rico formations

rhizoliths in ancient eolianites abstract rhizoliths are abundant within permo pennsylvanian eolian rocks of southeastern utah they are also present at the tops of several eolian sand bodies within the pennsylvanian fountain formation of southeastern wyoming and at a single stratigraphic level within the jurassic navajo sandstone of southeastern utah ranging in diameter from less than 1 mm to 15 cm these cylindrical structures are composed of siliciclastic free micrite and calcite spar cylinders ramify throughout the uppermost 2 3 m of thin flat topped sand bodies large cylinders are compound rhizoliths and display alveolar fabric a feature resulting from rootlet calcification that is characteristic of modern and ancient calcretes development of rhizoliths required flat geomorphically stable substrates for plant growth highly permeable surficial sediments and sources of calcium ions the restriction of rhizoliths to planar deflation surfaces suggests that plants were unable to colonize or stabilize eolian bedforms paleozoic rhizoliths appear to be restricted to coastal areas with onshore winds associated evaporites suggest an arid to semi arid paleoclimate rhizoliths formed when carbonate allochems derived from an upwind epeiric sea were dissolved and calcite reprecipitated around plant roots quaternary rhizoliths are best developed in similar sedimentologic and geographic settings the abundance of rhizoliths and carbonate allochems in quaternary and late paleozoic low latitude coastal dunes suggests that they are genetically related to glacio eustatic sea level fluctuations

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 architecture bedform climbing and preservation space in the cretaceous etjo formation nw namibia sets of aeolian cross strata within the cretaceous etjo formation of nw namibia are bounded by a hierarchy of surfaces the origin of which are ascribed to one of four processes related to aeolian bedform and erg behaviour the base of the main aeolian succession is characterized by a basin wide erosional supersurface that formed in response to a period of aeolian deflation before the onset of the main phase of erg building interdune migration surfaces formed by draa migration are planar in sections parallel to the palaeowind and are inclined at up to 5 in an upwind direction sw perpendicular to the palaeowind interdune surfaces form 500 m wide troughs signifying crestline sinuosity within the original bedforms superimposition surfaces are inclined at 5 10 in a downwind direction and indicate the migration of crescentic oblique dunes over larger slipfaceless transverse draa reactivation surfaces associated with minor changes in dune slipface orientation are distinct from other bounding surface types because overlying cross strata lie parallel to them rather than downlap onto them analysis of the geometry of these bounding surfaces together with the orientation of the cross strata within the sets that they bound has enabled the detailed morphology of the original bedforms to be reconstructed the maximum preserved thickness of individual aeolian sets varies systematically across the basin from 52 m in the basin depocentre to only 8 m at the basin margin the set architecture indicates that this spatial variation is primarily the result of decreased angles of bedform climb at the basin margin rather than the presence of smaller bedforms similarly a temporal reduction in the angle of climb rather than a reduction in bedform size is considered to be responsible for an upward decrease in preserved set thickness reductions in bedform climb angle reflect progressive loss of accommodation space as the accumulating erg filled the basin

stoss side mud drapes deposits of interdune pond margins arcuate mud drapes have been observed in an interdune area in southern tunisia they are believed to have been formed by deposition from ponded water around aeolian dunes during flood events they are preferentially preserved on the stoss side of the dunes as a result of subsequent dune migration and exhibit extensive evidence of animal tracks they are useful in illustrating dune interdune dynamics and recording the type of animals present

flood surfaces and deflation surfaces within the cutler formation and cedar mesa sandstone permian southeastern utah areally extensive erosion surfaces in eolian deposits have been interpreted as long hiatuses in eolian deposition such erosion surfaces form during deflationary episodes or during periods of erg stabilization or contraction they are called super surfaces and can be useful for stratigraphic correlation because of their regional extent the permian cedar mesa sandstone and cutler formation contain a type off super surface herein termed flood surfaces 10 400 km2 flood surfaces form as a result of fluvial floods into active dune seas the surfaces may expand through nonclimbing migration of eolian dunes but do not imply long hiatuses in eolian deposition as do other types of super surfaces cutler and cedar mesa flood surfaces are overlain by shales and sandstones which thicken laterally and merge with fluvial channel fill deposits flooding of active dune fields in the cedar mesa sandstone is suggested by intertonguing of eolian dune and fluvial deposits flood surfaces can easily be mistaken for deflationary super surfaces but are distinguished by evidence of dune migration coincident with flood events and by an increase in the number of surfaces adjacent to associated aqueous deposits

amalgamated interdune deposits white sands new mexico abstract cores taken by mckee and moiola 1975 from white sands dune field indicate that interdune areas are underlain by 7 0 10 4 m of clastic gypsum deposits clean sand bodies as much as 4 5 m thick are separated by thin beds of silly sand mckee and moiola interpreted the sequence to be the product of climbing bed forms interdune silty sands were interpreted to be deposited upon truncated dune sands and to be soon buried by the next wave of advancing dunes if the dunes are climbing in such a manner a pinchout of interdune sediments downwind across interdune surfaces is required the thickness of the silly sands should be related to the size of the interdune area and the rate of dune migration we tested this autocyclic hypothesis by trenching the length of the interdune areas in both t e barchanoid and transverse dunes trenches revealed that interdune deposits form flat lying sheets which extend uninterrupted to positions directly beneath the windward slopes of dunes at the downwind end of each interdune area this relationship precludes the possibility that under present conditions thick dune sands are being preserved because there is little or no net accumulation of dune sand interdune deposits are being amalgamated thin lenses of dune sediment exposed on the wall of one trench show that a minimum of three interdune deposits have been fused to form the uppermost 30 cm of the interdune deposit clean sand in the cores may have accumulated during periods when sediment supply was greater than at present silly sediments accumulated during periods when as in the p esent sand supply was limited and dunes did not climb appreciably future studies of modern and ancient eolian sediments should attempt to document whether the interdune deposits represent deposition within single interdune areas or amalgamation of several discrete deposits

significance of thin sets of eolian cross strata abstract thin cosets composed of thick sets of trough cross strata record episodes when ergs were sand saturated and erg buildup exceeded the rate of subsidence and water table rise interdunes were enclosed and dry thin sets with irregular set boundaries accumulated when ergs were unsaturated as at white sands dune field in southern new mexico interdunes were flat extensive and moist thin lenses of cross strata with irregular upper surfaces are the erosional remnants of lowermost portions of dunes that were within the zone of capillary rise thin flat bedded to massive interdune deposits were also preserved in this setting due to low sand supply upbuilding of the erg surface was limited to the rate of subsidence water table rise like sediments deposited in shallow subaqueous settings bu unlike most colian sands bed by bed accumulation of the thin sets was limited by short term accommodation space the allogenic interpretation presented above contrasts strongly with the widely accepted climbing interdune autogenie interpretation of the entrada we question this gradualistic interpretation not only on the basis of stratal relationships between dune and interdune deposits but on theoretical grounds as well to maintain moist wet interdunes when large bedforms are climbing to preserve thick tabular sand bodies deposition must not exceed the rate of groundwater rise we have seen very few irregular erosion surfaces between thick sets of eolian cross strata they are extremely common between thin sets widespread moist interdunes in modern ergs and evidence of high water tables in ancient eolian sequences are signals of low sand supply thin eolian sets with irregular upper b unding surfaces record the birth and demise of ergs as well as the fluctuations of erg margins the absence of animal tracks in the thick jurassic cross strata and their great abundance in and adjacent to the thin sets may be a result of an erg margin origin of the thin sets or of ameliorated climatic conditions during their deposition because they accumulate in zones and episodes of low sand supply cosets composed of thin sets are akin to deflation surfaces in for example the permian cedar mesa sandstone the movement of the groundwater table after the initial deflationary phase slow rise in the jurassic rapid drop in the permian may reflect the higher amplitude of late paleozoie sea level fluctuations