Aeolian Systems Research Articles

Holocene aeolian development in Central Spain; chronology, regional correlations and causal processes

Extensive areas in the southern part of the Duero Tertiary Basin (Central Spain) are covered by aeolian sands. Presently, the aeolian system is relict but in its origin and development it can be described as a “wet aeolian system”. Climatic and environmental changes during the Holocene are typified by alternating humid and arid periods. These are recorded in the sedimentary record as either organic-rich sandy palaeosols or clean aeolian sand, respectively. Palaeosol dating (12 radiocarbon dated samples) and stratigraphical and sedimentological analysis of several dunefields in quarries and boreholes allow the distinction of four periods of palaeosol development since the Allerød. Aeolian sediments commonly rest on fluvial deposits, which were themselves the major source area for aeolian sands. These fluvial deposits have an age of about 14,000 cal yr BP. The first phase of aeolian activity postdates these fluvial sediments and has an upper age of about 12,000–11,700 cal yr BP, probably corresponding to the last cold oscillation of the Lateglacial (Younger Dryas). The second phase ranges from about 11,500 to 9500 cal yr BP, during which period the majority of dunes in the Tierra de Pinares area formed. This is also a major phase of aeolian activity in other areas of the Iberian Peninsula. A third and probably discontinuous phase of aeolian activity took place between 6800 and about 3000 cal yr BP. The age for this phase is supported by the presence of Visigothic burial sites covered by aeolian sands. The presence of charred material and degraded slipfaces clearly indicate stabilisation by vegetation and the final degradation of the aeolian system at the end of the fourth aeolian phase (990–540 cal yr BP). Minor aeolian activity has also occurred subsequently in this area, since aeolian sand movement was even reported in the 20th century. The aeolian phases can be tentatively correlated with aeolian phases in Europe. Aeolian activity tends to occur regionally during specific time-intervals, especially in dunefields with little human disturbance. This argues for a broad climatic forcing in Holocene aeolian accumulation, such has been previously suggested for the little ice age. The precise timing of these phases, however, is not strictly coincident, probably due to the delayed responses of aeolian environments to climatic and subsequent vegetation change.

Cross-bedding set thickness and stratigraphic architecture of aeolian systems: An example from the Upper Permian Pirambóia Formation (Paraná Basin), southern Brazil

The Pirambóia Formation comprises an unconformity-bounded aeolian succession essentially composed of three facies associations: aeolian sand sheet, aeolian dune and interdune facies associations. The lower portion of the Pirambóia Formation is characterised by aeolian sand sheet deposits, which are overlain by aeolian dune and interdune strata, hence pointing to an overall increase in sand availability within the paleoerg. The dune and interdune successions can be further subdivided into two distinct stratigraphic intervals in terms of their mean set thickness. Intervals 1 and 2 display mean set thicknesses of 2.9 and 6.19 m, respectively. This increase in the mean set thickness reflects an increase of the angle of climb and/or dune size. In addition to improve the stratigraphic subdivision, the recognition and correlation of intervals with distinct mean set thicknesses provides a tool for reconstructing aeolian erg architecture from drill cores.

Point pattern analysis of north polar crescentic dunes, Mars: A geography of dune self-organization

The geographic signature of dune distribution and self-organization as measured by the R-statistic offers a viewpoint on the geography of crescentic eolian systems and proposes an index from which to determine the degree of self-organization across a variety of spatial scales. Fields of simple dunes (dome, barchan, barchan-seif) are comparatively less regular in distribution than are those fields, or part thereof, that consist of compound (barchanoid) morphologies whose patterns are more highly regular.

Formation of aeolian dunes on Anholt, Denmark since AD 1560: A record of deforestation and increased storminess

Sand dunes on the island of Anholt (Denmark) in the middle of Kattegat form a relatively barren, temperate climate aeolian system, locally termed the “Desert”. The dunes have developed on top of a raised beach ridge system under the influence of dominant winds from westerly directions. They are relatively coarse-grained with an average mean grain size of 480 μm. The last phase of aeolian activity and dune formation on Anholt started after AD 1560, when the local pine forest was removed. Historical sources report intense sand mobilization in the 17th century, and new optically stimulated luminescence (OSL) dates indicate that dune formation continued until the end of the 19th century. This period of sand drift and dune formation took place during the later part of the Little Ice Age, which is characterized by increased (summer) storminess in large parts of NW Europe. Dune stabilization in the beginning of the 20th century probably records a temporary decrease in storminess. Ground-penetrating radar mapping of the internal structures in two dunes in the western part of the Desert (a parabolic dune and a linear dune) indicates the importance of north-westerly (storm) winds during dune formation.

Eolian depositional episodes controlled by Late Quaternary relative sea level changes on the Imbituba–Laguna coast (southern Brazil)

The Late Quaternary coastal eolian deposits of the Imbituba–Laguna region (Santa Catarina State, southern Brazil) comprise two “eolian depositional sequences”, separated by an unconformity. The older eolian sequence, A, is correlated to the relative sea level rise of the Last Interglacial period. The eolian sequence B was initiated during the relative sea level rise in the post -glacial period and extends up to active dunefields. The correspondence between the two eolian depositional episodes represented by these sequences and the relative sea level highstand phases is confirmed by thermoluminescence and optically stimulated luminescence dating. This correspondence is explained by a conceptual model that links the availability of coastal sediments for eolian transport with relative sea level changes. The link between relative sea level changes and coastal eolian sedimentation can be used to integrate coastal eolian systems to the sequence stratigraphy model.

Evolution of Eureka Flat: A dust-producing engine of the Palouse loess, USA

Sedimentologic, stratigraphic, and pedologic evidence demonstrates that Eureka Flat (EF), a narrow, 80-km-long deflation plain in south-central Washington, USA, is the source for the thick and widespread loess of the Palouse region. Located in the southern Columbia Plateau, EF has been a depocenter for abundant clay-to-sand-sized sediment derived from repeated glacial outburst floods that swept through the area during the Pleistocene. Prevailing SW winds that were funneled across EF remobilized flood sediment into sand dunes, sand sheets, and loess. An eolian sand sheet buried beneath 1.8 m of loess at the downwind end of EF attests to bioclimatically driven changes in the style of eolian deposition. The depositional transition from sand sheet to loess is characterized by a change from wind-rippled strata to structureless, bimodal sand and silt with insect burrows and rhizoliths, to unimodal sandy loess. Soil moisture and density of dust-trapping vegetation control where sand dunes and loess accumulate. Up to 4.5 m of post-LGM loess mantles the landscape adjacent to and downwind from EF. Some of the thickest loess deposits owe their origins to the sequestering of eolian sand particles by topographic traps such as the Touchet River and its tributary valleys. The removal of saltating sand from the eolian system promotes thick accumulation of loess downwind of topographic traps. Patterns of loess accumulation across the Columbia Plateau are identified on isopach maps of major pre- and post-LGM loess units and show that EF has been the major engine for the production of atmospheric dust and loess during both times.

White Sands Dune Field, New Mexico: Age, dune dynamics and recent accumulations

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.

A model for the active origin and development of source-bordering dunefields on a semiarid fluvial plain: A case study from the Xiliaohe Plain, Northeast China

Source-bordering dunefields have been reported in some drylands of the planet, but scarcely in China where there are extensive drylands. This article reports them in China for the first time, and presents a model for their active origin and development on a semiarid fluvial plain by means of satellite image analyses and field investigations. Local- and regional-scale examples are chosen to analyze the spatial patterns of dunefields, as well as the relationships with the fluvial systems in the central part of Naiman Banner where the Jiaolai River runs, and the lower Laoha River, and the middle and lower Ulijimulun River (principal tributaries of the Xiliaohe River). The active origin and development of source-bordering dunefields can be divided into four stages in terms of the spatial patterns of dunefields and channel dynamics: Stage I — individual dunes on the downwind margins of river valleys where running water constantly erodes the steep slopes of valley and where the downwind slopes orient to local dominant winds; Stage II — individual local-scale dunefields formed by deflation of the steep valley slopes and extending antecedent dunes downwind, together with the downstream displacement of meanders; Stage III — individual large-scale dunefield belts along the downwind margins of river valleys formed through frequent lateral migrations of channel; Stage IV — regional-scale dunefields formed mainly by river diversions due to climatic changes or tectonic movements. On the one hand, it is the running water's lateral migration, especially meandering, that prepares suitable places for aeolian systems in terms of both wind flow fields and sand sources, and subsequently it can further cause separate local-scale source-bordering dunefields to link together as a regional-scale dunefield belt given sufficient time. On the other hand, diversions of the river are bound to occur following changing hydrologic regimes resulting from tectonic movements or significant climate change (at regional and millennium scales). As a result, when some dunefield belts as well as the adjacent channels are abandoned, new channels work elsewhere in the same way to actively form new source-bordering dunefields and even dunefield belts at a regional scale.

The Upper Jurassic (Kimmeridgian) fluvial–aeolian systems of the southern Neuquén Basin, Argentina

The Kimmeridgian Quebrada del Sapo Formation in the southernmost Neuquén Basin in Argentina represents a succession up to 40 m thick of coarse- to fine-grained fluvial deposits overlain by aeolian deposits. These fluvial–aeolian deposits reflect a significant palaeogeographic change in the basin and are related to a major, tectonically enhanced, relative sea-level fall. The fluvial section is dominated by braided-channel, fine-grained ephemeral, and sheetflood deposits. Aeolian facies are dominated by dune deposits, with minor sandsheet and interdune units. Changes in the nature of both fluvial and aeolian sedimentation within the studied area suggest a regional variability of accommodation/sediment supply conditions. The regional changes of the aeolian succession likely reflect different relative positions within a major erg. In the upwind margin of the erg, a shallow water table promoted water-lain sedimentation in interdune areas, whereas in the central parts of the erg, dry sediment accumulation took place above the regional water-table level. The vertical transition observed in the Quebrada del Sapo Formation, from fluvial to aeolian deposits, may be the result of a local climatic change to drier conditions due to the development of a climatic barrier imposed by growth of a magmatic arc to the west. Alternatively, the vertical transition could be related to a lowering of the water table associated with the compartmentalization of the basin during a period of low sea level.

Periodic accumulation and destruction of aeolian erg sequences in the Permian Cedar Mesa Sandstone, White Canyon, southern Utah, USA

AbstractThe 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.

Stratigraphic evolution of a fluvial–eolian succession: The example of the Upper Jurassic—Lower Cretaceous Guará and Botucatu formations, Paraná Basin, Southernmost Brazil

The Guará and Botucatu formations comprise an 80 to 120 m thick continental succession that crops out on the western portion of the Rio Grande do Sul State (Southernmost Brazil). The Guará Formation (Upper Jurassic) displays a well-defined facies shift along its outcrop belt. On its northern portion it is characterised by coarse-grained to conglomeratic sandstones with trough and planar cross-bedding, as well as low-angle lamination, which are interpreted to represent braided river deposits. Southwards these fluvial facies thin out and interfinger with fine- to medium-grained sandstones with large-scale cross-stratification and horizontal lamination, interpreted as eolian dune and eolian sand sheets deposits, respectively. The Botucatu Formation is characterised by large-scale cross-strata formed by successive climbing of eolian dunes, without interdune and/or fluvial accumulation (dry eolian system). The contact between the Guará and the Botucatu formations is delineated by a basin-wide deflation surface (supersurface). The abrupt change in the depositional conditions that took place across this supersurface suggests a major climate change, from semi-arid (Upper Jurassic) to hyper-arid (Lower Cretaceous) conditions. A rearrangement of the Paraná Basin depocenters is contemporaneous to this climate change, which seems to have changed from a more restrict accumulation area in the Guará Formation to a wider sedimentary context in the Botucatu Formation.

Vallecito Formation (Miocene): The evolution of an eolian system in an Andean foreland basin (northwestern Argentina)

The Vallecito Formation, a red bed sequence deposited during the Lower Miocene in the Andean foreland basins of the Precordillera (Argentina), has been studied to enable an analysis of the characteristics and temporal evolution of its facies and the major extrinsic factors that control its development. The unit is composed mainly of eolian sandstones and thin intercalations of fine-grained conglomerates and mudstones. The lithology, eolian stratification style, bounding surface type, cross-bedded set geometry, and depositional unit scale indicate six facies associations. Facies association I represents an eolian sand sheet; facies associations II, III, IV, and VI correspond to dunes and draas; and facies association V constitutes fluvial–eolian interaction deposits. With regard to sandstone composition, the Vallecito Formation is formed of feldlithic and lithic arenites with variable proportions of quartz and abundant volcanic rock fragments. The Vallecito Formation represents a large dune field developed in arid to semiarid climates during the first stages of the Andean orogeny. Its formation and development were controlled by climatic and tectonic factors. Concepts about eolian sediment states enable the establishment of a three-phase model of eolian evolution: (1) constructional, (2) maximum development, and (3) destructional. Contemporaneous influx in the constructional phase was limited by low sand availability in the marginal eolian sand sheets and the transport capacity of the wind in the main dune field. During the maximum development phase, dunes and draas grew under contemporaneous influxes limited by the transport capacity. The destructional phase corresponds to alluvial system progradation, which in some localities formed a fluvial–eolian interaction system where contemporaneous and lagged sediment influx was limited by sand availability. Eolian successions such as the ones represented by the Vallecito Formation are interpreted as foredeep deposits of continental foreland basins. In this depozone, the eolian facies correspond to very thick deposits characterized by thick cross-bedded sets, limited fluvial intercalations, a lack of paleosols, and high subsidence rates.

An experimental study on the susceptibility of crusted surfaces to wind erosion: A comparison of the strength properties of biotic and salt crusts

Penetrometry has been employed in a number of studies investigating the strength of crusted sediments as related to their stability to particle impact in aeolian systems. Despite the early success of this procedure, questions remain concerning the effect of the angle of penetration, the role of fatigue, and the relative importance of the ultimate strength as compared to the elasticity of the crust. This paper reports on a series of penetrometer and wind tunnel experiments that consider these effects in the context of both salt and biotic crusts. The results demonstrate that the angle of penetration is not important but the density of penetrations is. The decreased strength observed with high densities of penetration, repeated loading, and continual impact in wind tunnel experiments provides further confirmation that sedimentary crusts progressively become weaker during mass transport events. The elasticity of the surface appears to be at least as important to consider as the ultimate strength. Although much stronger, the salt crusts were found to break down and erode sooner than biotic crusts in wind tunnel studies. The notable elasticity of biotic crusts appears to afford them some protection against abrasion.

Unveiling past aeolian landscapes: A ground-penetrating radar survey of a Holocene coastal dunefield system, Thy, Denmark

The southern part of the Hvidbjerg coastal dunefield and the adjacent part of the Lodbjerg coastal dunefield system in northwestern Denmark has been mapped using ground-penetrating radar. Coastal cliff exposures and 14C datings indicate that the presently stabilized dunefield is underlain by a 10–15 m thick succession of Holocene aeolian sand deposits and interbedded peaty palaeosols. This succession is superimposed on a Weichselian till towards the south and a mid Holocene beach ridge and swash bar complex towards the north. The peaty palaeosols represent preceding periods of dunefield stabilization. These palaeosols reveal ages about AD 1100–1200, 700 BC and 2100 BC and act as marker horizons in the study area. More than 16 km of high-resolution GPR data covering an area of 6 km 2 were collected in the near-coastal part of the dunefields and enabled a correlation of these marker horizons across the dunefields. These data allowed the elaboration of topographic subsurface and isopach maps, and enabled an interpretation of the accumulation history of the aeolian system since about 2200 BC. This accumulation history tells of alternating periods with a stabilization of the dunefields and periods with high sand accumulation. Basically a perched ground water level and the morphology of the previous stabilized dunefield made it possible to generate an accumulation space large enough to preserve the sand during periods with stronger winds and marked aeolian activity. This 3D mapping of the stabilized dunefield subsurfaces has made it possible to unveil the development of the past aeolian landscape.

A Late Quaternary lizard assemblage from the Southern Pampean Region of Argentina

ABSTRACT Lizard remains are described from the late Quaternary Camping Americano locality, situated about 5 km south-southwest of Monte Hermoso (38°57′47″ S, 61°22′48″ W) in Buenos Aires province, Argentina. Remains consist of 40 dentary fragments, 15 partial maxillae, and one vertebra. Two liolemids (Liolaemus multimaculatus and Liolaemus darwinii), one gekkonid (Homonota sp.), and one teiid (Cnemidophorus sp.) are recognized. Only the first two species live in the Camping Americano area at present. Divergence of the Liolaemus multimaculatus group might have taken place during development of the late Quaternary Pampean Aeolian System. Climate change in the southern Pampean Region during the last several thousand years might explain the disappearance of gekkonid and teiid taxa from the Camping Americano area.

GYR, A. & KINZELBACH, W. (eds) 2003. Sedimentation and Sediment Transport. Proceedings of the Symposium held in Monte Verità, Switzerland, from September 2nd to September 6th, 2002. x+271 pp. Dordrecht, Boston, London: Kluwer Academic Publishers. Price Euros 115, US $113, £72 (hard covers). ISBN 1 4020 1266 7

In order to complete reviewing Sedimentation and Sediment Transport , I first had to prise the book away from several of my colleagues, proof indeed that this book is of great interest to Earth Scientists. The book sets out to address what the editors see as a stagnation of research in the fields of sedimentation and sediment transport over the last several decades, and provides a series of papers from a symposium held in Switzerland in September 2002 that brought together workers from across mathematics and the natural sciences. The breadth of the text is impressive, from theoretical studies of turbulence, through physical experiments, to field studies; and with applications including rivers, aeolian systems, estuaries, coastal zones and gravity currents, as well as many purely generic studies. In total, there are some 41 contributions, with an average paper length of under …

Eustatic and climatic controls on the development of the Wahiba Sand Sea, Sultanate of Oman

AbstractAeolian sand sea accumulations can serve as valuable archives of climate change in continental environments. The Wahiba Sand Sea is situated at the northern margin of the area presently affected by Indian Summer Monsoon Circulation and it records environmental changes associated with this major climatic boundary over the last 160 000 years. The internal stratigraphy and evolution of the sand sea is investigated using a combination of outcrop, borehole, seismic and luminescence data. Proximity to the Indian Ocean means that the sand sea succession shows the influence of sea level changes on the sedimentary architecture and composition of the dune deposits. During the last two glacial periods, low global sea level was associated with a high input of bioclastic grains, reflecting the significance of subaerially exposed shelf areas as one of the main sources of aeolian sediment. The onset of aeolian sediment transport and deposition was related to the breakdown of stabilizing vegetation during arid periods that equate with sea level lowstands. The preservation of aeolian sediments by the formation of supersurfaces and associated palaeosoils took place during times of increased wetness and elevated groundwater tables. This interplay of constructive and destructive periods greatly influenced the sedimentary architecture. Oscillations of wet and dry periods between 160 000 and 130 000 years and 120 000–105 000 years ago are attributed to the evolution of a wet aeolian system. Younger periods of aeolian deposition around and after the last glacial maximum were characterized by dry aeolian conditions. No soil horizons developed during these times.

Reconstitution of dune morphologies and palaeowind regimes in the Proto-Namib since the Miocene

Abstract In the Namib Desert, indurated fossil dunes are found between the active sands. Establishment of a biostratigraphic scale based on associated mammal faunas and ratite eggshells allows the study of palaeowind fluctuations that have controlled these palaeodunes since the Middle Miocene with a temporal resolution of between 1 and 3 Ma. In the southern Namib, the Proto-Namib aeolian systems consist of crescent dunes (barkhan type). In the eastern part of the Namib Desert, the aeolianite cross-bedding corresponds to longitudinal or pyramidal dunes. These aeolian structures are also controlled by local winds, resulting from high relief and the southern trade winds : this regime is identical to that which currently sweeps, through the Namib Desert, and is related to the presence of anticyclonic conditions in the Atlantic Ocean. A regime of seasonal winds from the northeastern sector is also recorded starting in the late Miocene.

Spatial and temporal analysis of crust deterioration under particle impact

Recent work by Rice et al. (Earth Surf. Process. Landforms 21 (1996) 279; Earth Surf. Process. Landforms 22(9) (1997) 859; Earth Surf. Process. Landforms 26 (2001) 721), and McKenna Neuman and Maxwell (Catena 38(2) (1999) 151; Earth Surf. Process. Landforms 27 (2002) 891) has contributed much to the measurement and understanding of crust strength in relation to the impact force of particles striking a crusted surface. However, the spatial and temporal aspects of crust destruction under impact remain largely unknown. The present paper expands the analysis of data obtained from an earlier wind tunnel simulation of crust destruction (McKenna Neuman and Maxwell, 2002). A detailed spatial and temporal analysis is presented for over 4000 sites of crust loss, as preserved in photographic images that have not been previously considered. The results suggest that the fractal dimension of the digitized deflation features falls between 1.09 and 1.24, is consistent for any given crusted surface, and may be positively correlated with independent measurements of crust strength. Small erosion pits (<10mm2) can appear at any time, even after an hour or more of impact. This observation reinforces our earlier suggestion (McKenna Neuman and Maxwell, 2002) that the duration of impact is significant and that a fatigue-like mechanism is likely at work. While the orientation of any given deflation feature can vary through 180°, analysis of a large sample (n=4100) shows that on average and regardless of size, they are aligned with the air stream. The results from this study present a number of possible simplifications for those interested in modeling these aeolian systems in future.

Stratigraphic evolution of an aeolian erg margin system: the Permian Cedar Mesa Sandstone, SE Utah, USA

AbstractThe 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.