Aeolian Bedforms Research Articles

Early‐stage aeolian protodunes: Bedform development and sand transport dynamics

AbstractEarly‐stage aeolian bedforms, or protodunes, are elemental in the continuum of dune development and act as essential precursors to mature dunes. Despite this, we know very little about the processes and feedback mechanisms that shape these nascent bedforms. Whilst theory and conceptual models have offered some explanation for protodune existence and development, until now, we have lacked the technical capability to measure such small bedforms in aeolian settings. Here, we employ terrestrial laser scanning to measure morphological change at the high frequency and spatial resolution required to gain new insights into protodune behaviour. On a 0.06 m high protodune, we observe vertical growth of the crest by 0.005 m in two hours. Our direct measurements of sand transport on the protodune account for such growth, with a reduction in time‐averaged sediment flux of 18% observed over the crestal region. Detailed measurements of form also establish key points of morphological change on the protodune. The position on the stoss slope where erosion switches to deposition is found at a point 0.07 m upwind of the crest. This finding supports recent models that explain vertical dune growth through an upwind shift of this switching point. Observations also show characteristic changes in the asymmetric cross‐section of the protodune. Flow‐form feedbacks result in a steepening of the lee slope and a decline in lower stoss slope steepness (by 3°), constituting a reshaping of protodune form towards more mature dune morphology. The approaches and findings applied here, (a) demonstrate an ability to quantify processes at requisite spatial and temporal scales for monitoring early‐stage dune evolution, (b) highlight the crucial role of form‐flow feedbacks in enabling early‐stage bedform growth, alluding to a fluctuation in feedbacks that require better representation in dune models, and (c) provide a new stimulus for advancing understanding of aeolian bedforms. Copyright © 2017 John Wiley & Sons, Ltd.

Aeolian saltation on Mars at low wind speeds

AbstractLaboratory experiments indicate that the fluid threshold friction speed, u*tf, required to initiate fully developed aeolian saltation is much higher on Mars than on Earth. A discrepancy exists between Mars climate models that do not predict winds this strong and observations that sand‐sized particles are indeed moving. This paper describes how wind friction speeds well below u*tf, but above the impact threshold, u*ti, required to sustain saltation, can initiate sustained saltation on Mars, but at relatively low flux. Numerical experiments indicate that a sand grain on Mars mobilized sporadically between u*ti and u*tf will develop, over fetch lengths longer than generally available within low‐pressure wind tunnels, trajectories capable of splashing grains that propagate saltation and collectively form a cluster of saltating grains that migrate downwind together. The passage of a saltation cluster should leave behind a narrow zone of affected surface grains. The cumulative effect of many clusters represents a low‐flux phenomenon that should produce slow changes to aeolian bedforms over periods in which winds remain close to u*ti and never or rarely reach u*tf. Field evidence from small impact ripples along rover traverses is consistent with effects of saltation at these low friction speeds, without obvious evidence for events ≥u*tf. The potential utility of this grain mobility process is that it can operate entirely at more common winds well below u*tf and so help explain widespread sand movements observed on Mars wherever evidence might be mostly absent for u*tf being exceeded.

Wind-pattern circulation as a palaeogeographic indicator: Case study of the 1.5–1.6 Ga Mangabeira Formation, São Francisco Craton, Northeast Brazil

The preserved deposits of dune-scale aeolian bedforms provide valuable palaeoenvironmental indicators of atmospheric circulation patterns and the latitudinal position and distribution of land masses. However, no attempts to use palaeowind directions and palaeogeographic reconstructions of ancient land mass distribution have been published to model Precambrian atmospheric circulation. The Mangabeira Formation is a large Mesoproterozoic aeolian erg succession (1.6–1.5Ga) composed of two aeolian units that accumulated in the São Francisco Craton, Brazil. The Lower Unit records multiple drying-upward depositional cycles, each of which represents an episode of erg expansion and contraction driven by climate changes. The Upper Unit is composed dominantly of stacked aeolian dune strata that lack intervening interdune deposits and which record extreme aridity. Palaeowind directions recorded from cross-strata of transverse, crescentic aeolian dunes of the Lower and Upper Units record dune migration under the influence of two dominant winds that blew to the southeast and northwest. Analysis of these palaeowind data in relation to assessment of regional palaeogeographic reconstructions for the period 1.6–1.5Ga reveals a correlation between atmospheric circulation and land mass distribution. At this time the São Francisco Craton was located between the mid-latitudes and the equatorial zone. The wind regime determined from analysis of dip azimuths of cross-strata of the Lower Unit (1.6–1.54Ga) are consistent with a palaeogeographic position between 25° and 35° S. Analysis of cross-strata dip azimuths of the Upper Unit indicate northwest-directed palaeowinds and a dominant monsoonal wind pattern from 1.54 to 1.5Ga. During this time the large land mass of the São-Francisco-Congo and Siberian cratons drifted northwards through the equatorial zone from palaeolatitude 30° S to 30° N.

Holocene fluvial valley fill sources of atmospheric mineral dust in the Skeleton Coast, Namibia

AbstractWestern Namibia is a significant global source of atmospheric mineral dust. We investigate the relationship between dust and source sediments, assessing the sustainability of dust flux. Remote sensing studies have highlighted specific ephemeral fluvial systems as important contributors to dust flux, including highlighting sections of valleys that are the origins of dust plumes in the period 2005–2008. Little is known however about the specific within‐valley dust sediment sources, particularly whether dust is derived from modern ephemeral channel floors or older valley fill sediments, many of which have been reported in the region. As part of a region‐wide analysis of aeolian dust flux, we investigate the sediment properties of atmospheric dust samples and valley sediments from the Huab valley, one of the principal regional dust sources. Trapped dust samples contain up to 88% very fine sand and silt when collected samples are disaggregated prior to analysis. Valley fill surface samples comprise 80% very fine sand and silt, and the surface of the modern ephemeral channel 30%. Valley fill sediments were sampled at depths up to 3.6 m below the present surface and reveal Holocene depositional ages from 0.6 ± 0.03 ka back to 9.79 ± 0.73 ka. These sediments contain 30% to 6% very fine sand and silt, with levels decreasing with depth and age. Aeolian bedforms in the valley system (nebkhas on the fill surface and climbing dunes on valley margins) indicate that aeolian processes under the influence of strong seasonal easterly winds likely result in dust being winnowed out of the valley fill surfaces, with sandy bedforms being constructed from the coarser component of the fill sediments. The volume of valley fill sediment suggests dust sourced from Holocene sediments is likely to continue into the future regardless of flow conditions in the modern channel system. Copyright © 2017 John Wiley & Sons, Ltd.

Terrestrial subaqueous seafloor dunes: Possible analogs for Venus

Abstract Dunes on Venus, first discovered with Magellan Synthetic Aperture Radar (SAR) in the early 1990s, have fueled discussions about the viability of Venusian dunes and aeolian grain transport. Confined to two locations on Venus, the existence of the interpreted dunes provides evidence that there could be transportable material being mobilized into aeolian bedforms at the surface. However, because of the high-pressure high-temperature surface conditions, laboratory analog studies are difficult to conduct and results are difficult to extrapolate to full-sized, aeolian bedforms. Field sites of desert dunes, which are well-studied on Earth and Mars, are not analogous to what is observed on Venus because of the differences in the fluid environments. One potentially underexplored possibility in planetary science for Venus-analog dune fields could be subaqueous, seafloor dune fields on Earth. Known to the marine geology communities since the early 1960s, seafloor dunes are rarely cited in planetary aeolian bedform literature, but could provide a necessary thick-atmosphere extension to the classically studied aeolian dune environment literature for thinner atmospheres. Through discussion of the similarity of the two environments, and examples of dunes and ripples cited in marine literature, we provide evidence that subaqueous seafloor dunes could serve as analogs for dunes on Venus. Furthermore, the evidence presented here demonstrates the usefulness of the marine literature for thick-atmosphere planetary environments and potentially for upcoming habitable worlds and oceanic environment research program opportunities. Such useful cross-disciplinary discussion of dune environments is applicable to many planetary environments (Earth, Mars, Venus, Titan, etc.) and potential future missions.

The nature and emplacement of distal aqueous‐rich ejecta deposits from Hale crater, Mars

AbstractHale crater formed in the Early to Middle Amazonian and is one of the best preserved large craters on Mars. We focus on the emplacement of previously mapped distal continuous ejecta and newly recognized discontinuous distal ejecta deposits reaching up to 450 km northeast of Hale. The distal continuous ejecta deposits are typically tens of meters thick, likely water‐rich, and subsequent dewatering of some resulted in flow along gradients of 10 m km‐1 for distances of tens of kilometers. The discontinuous distal ejecta are typically <10 m thick with volumes generally <0.5 km3 and embay Hale secondaries, which occur up to ~600 km from Hale. Both continuous and discontinuous distal ejecta deposits are typically smooth at scales of tens to hundreds of meters, relatively dark‐toned, devoid of eolian bedforms, inferred to be mostly fine‐grained, and were likely emplaced within hours to 1–2 days after impact. The occurrence of well‐preserved discontinuous distal ejecta at Hale is unusual compared to other large Martian craters and could be due to impact into an ice‐rich substrate that enabled their formation and (or) their survival after minimal postimpact degradation relative to older craters. The pristine nature of distal continuous and discontinuous distal deposits at Hale and the preservation of associated secondaries imply (1) low erosion rates after the Hale impact, comparable to those estimated elsewhere during the Amazonian; (2) the impact did not significantly influence long‐term global or regional scale geomorphic activity or climate; and (3) the Hale impact occurred after late alluvial fan activity in Margaritifer Terra.

Measuring (subglacial) bedform orientation, length, and longitudinal asymmetry – Method assessment

Geospatial analysis software provides a range of tools that can be used to measure landform morphometry. Often, a metric can be computed with different techniques that may give different results. This study is an assessment of 5 different methods for measuring longitudinal, or streamlined, subglacial bedform morphometry: orientation, length and longitudinal asymmetry, all of which require defining a longitudinal axis. The methods use the standard deviational ellipse (not previously applied in this context), the longest straight line fitting inside the bedform footprint (2 approaches), the minimum-size footprint-bounding rectangle, and Euler’s approximation. We assess how well these methods replicate morphometric data derived from a manually mapped (visually interpreted) longitudinal axis, which, though subjective, is the most typically used reference. A dataset of 100 subglacial bedforms covering the size and shape range of those in the Puget Lowland, Washington, USA is used. For bedforms with elongation > 5, deviations from the reference values are negligible for all methods but Euler’s approximation (length). For bedforms with elongation < 5, most methods had small mean absolute error (MAE) and median absolute deviation (MAD) for all morphometrics and thus can be confidently used to characterize the central tendencies of their distributions. However, some methods are better than others. The least precise methods are the ones based on the longest straight line and Euler’s approximation; using these for statistical dispersion analysis is discouraged. Because the standard deviational ellipse method is relatively shape invariant and closely replicates the reference values, it is the recommended method. Speculatively, this study may also apply to negative-relief, and fluvial and aeolian bedforms.

Our evolving understanding of aeolian bedforms, based on observation of dunes on different worlds

Dunes, dune fields, and ripples are unique and useful records of the interaction between wind and granular materials – finding such features on a planetary surface immediately suggests certain information about climate and surface conditions (at least during the dunes’ formation and evolution). Additionally, studies of dune characteristics under non-Earth conditions allow for “tests” of aeolian process models based primarily on observations of terrestrial features and dynamics, and refinement of the models to include consideration of a wider range of environmental and planetary conditions. To-date, the planetary aeolian community has found and studied dune fields on Mars, Venus, and the Saturnian moon Titan. Additionally, we have observed candidate “aeolian bedforms” on Comet 67P/Churyumov-Gerasimenko, the Jovian moon Io, and – most recently – Pluto. In this paper, we hypothesize that the progression of investigations of aeolian bedforms and processes on a particular planetary body follows a consistent sequence – primarily set by the acquisition of data of particular types and resolutions, and by the maturation of knowledge about that planetary body. We define that sequence of generated knowledge and new questions (within seven investigation phases) and discuss examples from all of the studied bodies. The aim of such a sequence is to better define our past and current state of understanding about the aeolian bedforms of a particular body, to highlight the related assumptions that require re-analysis with data acquired during later investigations, and to use lessons learned from planetary and terrestrial aeolian studies to predict what types of investigations could be most fruitful in the future.

A Surface Model for Aeolian Dune Topography

A surface model for aeolian bedform topography is adapted from a surface model of subaqueous bedform topography. The aeolian bedform surface model is developed using a uniform grid with a cell-centered finite volume approximation of the sediment continuity equation. The resulting modeling framework approximates the dynamic motions of aeolian bedform topography driven by bedform field boundary conditions. The numerical model is applied to simulate bedforms growing from unimodal and bimodal transport regimes from both a fixed elevation (sediment source area) and within a domain with fully periodic boundary conditions. The rates at which modeled aeolian bedforms grow and morphologically mature are sensitive to the chosen boundary conditions. Video files of model simulations and source code for the presented aeolian bedform surface modeling framework are available in supplemental materials. The aeolian bedform surface model code is malleable and readily modified for exploratory study of dynamic bedform topography that inherits morphological traits from aeolian bedform field boundary conditions.

A terrestrial analog for transverse aeolian ridges (TARs): Environment, morphometry, and recent dynamics

Transverse aeolian ridges (TARs) are a distinct aeolian bedform found on Mars. The formative processes, evolution, and geological significance of TARs is poorly understood. Fundamentally, it is unknown whether TARs are dunes, mega-ripples, or another bedform type. We examined aeolian bedforms in the Lut Desert of Iran as a terrestrial analog for Martian TARs. From an objective sampling strategy with high-resolution satellite imagery, we developed a large morphometric dataset for comparison with existing Martian TARs. We also examined the dynamics of the Lut bedforms between 2004 and 2012 to determine if they were static or migrating. Results indicate that the range in the dimensions (length, width, height, and wavelength) of the Lut bedforms and Martian TARs overlap, suggesting Lut bedforms are a viable terrestrial TAR analog. Our sample yielded median values of 55.18m, 9.80, 1.02m, and 20m for length (longest planview axis), width (shortest planview axis), height, and wavelength, respectively. Cumulative log-frequency plots of morphometric parameters suggest the sample is from a single population and process mechanism. Although the vast majority of Lut bedforms examined were static between 2004 and 2012, some migrated up to 0.09myr−1 on average. This is much slower than nearby dunes (4–12myr−1), but is explained by the existence of a surface lag of coarse particles on the TAR-like bedforms. The combination of morphometry, surface sedimentology, and slow migration rate indicate the Lut bedforms are mega-ripples, which provides evidence supporting interpretation of Martian TARs as mega-ripples. Testing the mega-ripple hypothesis for Martian TARs requires measurements of their sedimentology, which may be possible with the Mars Science Laboratory rover Curiosity, as well as expanded measurements of TAR morphometry to constrain their size, shape, and scaling.

Environmental dynamics of a star dune

Star dunes, the largest aeolian bedforms in the sand seas of the world, are usually distributed within specific geographical areas that have multi-directional wind regimes. However, relatively few studies have focused on the environmental factors that impart such great volumes of sand to these dunes. Specifically, verification of the developmental processes of star dunes through long-term monitoring is scarce. In this study, by observing 3-D airflow fields and long-term dune dynamics, we demonstrate how topographic barriers, which generate vertical airflow and local air circulation, control the development of a star dune on Mingsha Mountain in Dunhuang, China. Results show that airflow stagnation and deflection caused by topography is one of the major mechanisms for the formation of star dunes. In our study, topographic barriers contribute to the development of intensive vertical airflow dominated by easterly winds. This intensive vertical airflow is one of the main driving mechanisms of the upward growth of mega-dunes. Vertical airflow is the strongest developed airflow reported in available data on aeolian geomorphology. In addition, star dunes are usually distributed in areas where the local air circulation is strong. The results of long-term dune dynamics verify that local air circulation, which forms three wind directions with the regional wind regime, contributes to the maintenance and development of star dunes. Our study indicates that complex mega-dunes are products of topographic barriers, which facilitate their recognition in aeolian geomorphology. We introduce a new evolution pattern of star dunes under the influence of local environment and topographic barriers.

A sedimentary model for early Palaeozoic fluvial fans, Alderney Sandstone Formation (Channel Islands, UK)

Fluvial fans in the rock record are inferred based on critical criteria such as: downstream grain-size fining; evidence for drainage fractionation along bifurcating channels; increasing fluvial-aeolian interaction in the basinward direction; and radial palaeoflow dispersion. Since pre-vegetation fluvial rocks often lack heterolithic alluvium and channelisation at the outcrop scale, the recognition of pre-Silurian fluvial fans has, so far, not been straightforward. This research proposes a sedimentary model for the Alderney Sandstone Formation of Channel Islands (UK), so far considered as a fine record of early Palaeozoic axial-fluvial sedimentation. Here, outcrop-based and remote-sensing analysis of the formation's type-section reveal the interaction of fluvial and aeolian processes, expressed by the alternation of: compound fluvial bars enclosing macroform surfaces, related to phases of perennial discharge; fluvial sandsheets containing antidunal forms and soft-sediment deformations, related to seasonal (i.e. flashy) discharge; and aeolian bedforms overlying thin stream-flow deposits.An up-section increase in aeolian deposits is accompanied by the shrinking of fluvial bars and minor-channel cuts, suggesting that drainage was fractioned along smaller channels terminating into marginal aeolian environments. Together with a propensity towards more dispersed values of fluvial cross-set thickness up-section (again due to discharge fractionation along intermittently active channels), these features depict an aeolian-influenced fluvial fan. This work discusses a set of criteria for the identification of fluvial fans in pre-vegetation environments. In doing so, it also explores possible parallels to modern environments, and underscores the potential of integrated outcrop and remotely sensed observations on ancient fluvial rocks and modern sedimentary realms.

Aeolian sand transport and aeolian deposits on Venus: A review

Abstract We review the current state of knowledge about aeolian sand transport and aeolian bedforms on planet Venus. This knowledge is limited by lack of observational data. Among the four planetary bodies of the Solar System with sufficient atmospheres in contact with solid surfaces, Venus has the densest atmosphere; the conditions there are transitional between those for terrestrial subaerial and subaqueous transport. The dense atmosphere causes low saltation threshold and short characteristic saltation length, and short scale length of the incipient dunes. A few lines of evidence indicate that the typical wind speeds exceed the saltation threshold; therefore, sand transport would be pervasive, if sand capable of saltation is available. Sand production on Venus is probably much slower than on the Earth; the major terrestrial sand sinks are also absent, however, lithification of sand through sintering is expected to be effective under Venus’ conditions. Active transport is not detectable with the data available. Aeolian bedforms (transverse dunes) resolved in the currently available radar images occupy a tiny area on the planet; however, indirect observations suggest that small-scale unresolved aeolian bedforms are ubiquitous. Aeolian transport is probably limited by sand lithification causing shortage of saltation-capable material. Large impact events likely cause regional short-term spikes in aeolian transport by supplying a large amount of sand-size particles, as well as disintegration and activation of older indurated sand deposits. The data available are insufficient to understand whether the global aeolian sand transport occurs or not. More robust knowledge about aeolian transport on Venus is essential for future scientific exploration of the planet, in particular, for implementation and interpretation of geochemical studies of surface materials. High-resolution orbital radar imaging with local to regional coverage and desirable interferometric capabilities is the most effective way to obtain essential new knowledge about aeolian transport on Venus.

Mega-ripples in Iran: A new analog for transverse aeolian ridges on Mars

A new terrestrial analog site for transverse aeolian ridges (TARs) is described in this study. The Lut desert of Iran hosts large ripple-like aeolian bedforms, with the same horizontal length scales and patterns of TARs on Mars. Different classes of TARs and different types of other aeolian features such as sand dunes, zibars, dust devil tracks and yardangs can be found in this area, which signify an active aeolian region. This area represents a unique site to study the formation and evolution of these enigmatic features, with potential relevance toward a better understanding of TARs on Mars.

Deflated rims along the Xiangshui River on the Xiliaohe Plain, Northeast China: A case of active fluvial–aeolian interactions

Riverine source-bordering sand dunes, as a result of active fluvial–aeolian interactions, are a pronounced feature on the semiarid Xiliaohe Plain, Northeast China. By means of satellite imagery analysis, and both field survey and observation, this paper presents a new type of riverine source-bordering sand dunes — deflated rims, on the downwind margins of the Xiangshui River. They largely result from the deflation of escarpments on the downwind side of valley by local prevailing winds of NW direction, not from the reworking of point bars on floodplain by wind. In general, a rim is primarily composed of three distinct zones: 1) the upwind frontal escarpment zone with variable plan-form shape, gradient and relief, which is formed by either active lateral erosion by river or significant erosion by wind and transient slope runoff; 2) the deflation zone with gentle slopes of 8–18° and small-scale aeolian bedforms, i.e. ripples of fine sand, ridges of coarse sand; and 3) the downwind dynamic deposition zone with distinctive bedforms with variable superficial texture and slip faces. The sand mass on rims derives overwhelmingly from underlying loose late Quaternary sediments, is sufficient and sustainable by successive retreats of the escarpment, and is gradually transported downwind by pulse motions of bedforms, coupled with high wind events. Essentially, deflated rims are a starting point and the incipient phase of mature riverine dunefields. The superimposed bedforms on rims are fundamentally governed by windflow dynamics, sand sediments and antecedent bedform, exhibiting in turn the manner and intensity of rim development. Consequently, the upwind river valley and downwind deflated rim can jointly stimulate marked wave-like motion of both windflow and aeolian bedforms at different scales, especially when high wind events occur. This study sheds some light on the understanding of the origin and development of riverine source-bordering dunefields, and offers new insights into the mechanisms of fluvial–aeolian interactions on drylands worldwide.

Morphological and sedimentary features of oblique zibars in the Kumtagh Desert of Northwestern China

Zibars are coarse-grain-armored, low-relief aeolian bedforms with a large spacing but without slipfaces. Little is known of their morphodynamic processes, even though they are ubiquitous at upwind desert margins. In this paper, we describe the morphological and sedimentary features of oblique zibars in the northeastern Kumtagh Desert of China; analyze the effective wind regime; and discuss the significance of the coarse-grained sediments for zibar morphology and sedimentary structures. Due to a mantle of coarse grains, oblique zibars and nearby linear dunes respond to different effective wind regimes that result in the angle between their geomorphological trends. Topographic surveys showed that the spacing of the oblique zibars ranged between 130.5 and 357.5m, and that the height ranged between 0.6 and 3.0m. The oblique zibar surface is almost entirely covered by coarse sand, and includes gravel with a particle diameter of up to −2.34φ. The poorly sorted, positively to very positively skewed surface sediments, with mean grain size ranging between −0.2φ and 1.5φ, were characterized by a bimodal distribution. The spacing between the oblique zibars is closely related to the coarsest 5th percentile of the surface sediments, and can potentially be explained by a combination of the saturation length and minimum dune size theories. In addition, the protectionist theory, combined with the surface creep and vibration of coarse grains (which allows infiltration of finer materials between the grains), may account for the alternating coarse and fine laminae in the oblique zibars.

Reconstructing the aqueous history within the southwestern Melas basin, Mars: Clues from stratigraphic and morphometric analyses of fans

New details of the aqueous history in the southwestern Melas Chasma elevated basin have been revealed from analysis of high-resolution image, topographic and spectral datasets. We have identified eleven fan-shaped landforms that reflect various depositional environments. A distinctive marker bed with inferred indurated aeolian bedforms is within the stratigraphic record of presumed lacustrine deposits. This observation, taken together with the stratigraphic succession of fan-shaped deposits indicates fluctuating lake levels with, at a minimum, early and late-stage lake highstands. Tributary drainage pattern in the western valley network changed from a dendritic to a meandering system, recording a shift in fluvial activity that is consistent with fluctuating lake levels. Only a few hydrated minerals have been detected in the study region, the most common being opal which appears to represent younger alteration and deposition within the basin. Landform scale was used to estimate average discharge (∼30m3/s), formative discharge (200–300m3/s), and fan formation timescale, which further inform the duration of lacustrine activity within the basin. Warm surface conditions and precipitation recharge of source basins is required to generate and sustain this long-lived lake over periods of at least centuries to millennia during the Late Hesperian to Early Amazonian.

Persistent aeolian activity at Endeavour crater, Meridiani Planum, Mars; new observations from orbit and the surface

Aeolian-driven bedform activity is now known to occur in many regions of Mars, based on surface and orbital observation of contemporary martian ripple and dune mobility events. Many of these sites have only been monitored with sufficient resolution data for the last few Mars years, when the High Resolution Imaging Science Experiment (HiRISE) began acquiring images of Mars. One exception is the well-monitored Endeavour crater in Meridiani Planum, which was one of the first known sites of unambiguous dune activity (migration and deflation). However, those early detections used lower resolution images over longer temporal baselines (versus the HIRISE data now available), leaving some measurements poorly constrained. New orbital and surface observations of Endeavour show multiple spatial (cm, m, km) and temporal (seasons, Mars year) scales of aeolian-driven surface change, which confirms earlier reports. Dome dunes in the eastern portion of the crater persistently deflate, disseminating dark sand across lighter-toned regolith and/or eroded bright dust, and likely contribute to the crater interior’s episodic decreases in orbital albedo measurements. Other dome dunes are detected with the highest migration rates (4–12m per Mars year) and volumetric sand fluxes reported yet for Mars. Estimated dune construction times or “turnover times” here and elsewhere on Mars are significantly shorter than martian obliquity cycles, implying that it is not necessary to invoke paleoclimate wind regimes to explain current dune morphologies. Located on the crater rim, the Opportunity rover detected evidence for near- and far-field aeolian-driven activity, with observations of spherules/sand movement in the rover workspace, bedform albedo alteration, and dust-lifting events. Observations of intracrater dunes show periodic shifting dark streaks that significantly constrain local wind regimes (directionality and seasonality). Constraints on wind directions from surface and orbital images show that aeolian bedforms can be extremely active in bi-, and possibly tri-, modal wind regimes, and during periods without major dust storms.

Palaeoproterozoic fluvio-aeolian deposits from the lower Gulcheru Formation, Cuddapah Basin, India

An analysis of facies was done to understand the depositional environment and the palaeoclimate of the sedimentary succession from the lower part of the Palaeoproterozoic (∼2.0Ga) Gulcheru Formation exposed along the southwestern margin of the Cuddapah Basin. Twelve distinct sedimentary facies were identified and grouped into three main facies associations – wadi fan, ephemeral fluvial and aeolian. Identification of the fluvial and the aeolian facies allowed a more elaborate interpretation of the depositional environment and its palaeoclimate. Facies characteristics indicated that the sediments in the beginning were deposited in a dominantly aeolian realm, under warm and semiarid climatic condition. Translatent strata, pin stripe lamination, zibars, high-index granule ripples, sand sheet deposits, grainflow cross-strata and grainfall laminae, asymptotically down-lapping cross-strata often with erosional lower bounding surface and massive sand-bodies with bimodal fabric, the unambiguous evidences of aeolian depositional regime led to this conclusion. However, the aeolian regime was often punctuated temporarily by fluvial input from ephemeral streams during sudden rainstorm. Depending upon the size, character and availability of sediments, relief difference and the sediment/water ratio cohesionless debris flow, hyperconcentrated flood flow and sheetflood deposits were formed near the basin margin, whereas, coarse-load braided channel deposits were laid further inside the basin. Ephemeral lakes/ponds were formed due to stagnation of floodwater in normally dry interdune lows. Overbank-interdune sediments were deposited in those ephemeral lakes/ponds. Amongst the aeolian facies, translatent strata and sand sheet dominate in the west, whereas, massive beds and dunes with well-developed slipfaces dominate in the eastern part. The spatial distribution of the aeolian bedforms suggests development of erg apron to the west and dune field (erg) to the east. The aeolian sediments identified in the Gulcheru Formation may be considered to be amongst the oldest Palaeoproterozoic aeolian sediments of the world.

Planetary dune analogues: a special issue on integrating models, remote sensing and field data

ABSTRACTThis article reports on the Special Issue of Earth Surface Processes and Landforms dedicated to planetary dune analogues and the advances that have been made through the integration of remote sensing and terrestrial field data to better model and understand aeolian systems. The 11 articles presented demonstrate a spectrum of analytical techniques that are employed to investigate ripples, dunes, draa and ergs across a variety of spatial and temporal scales. Nine of the papers involve innovative research about aeolian bedforms and processes, while the final two papers are examples of how vast quantities of data may be organized and scrutinized using a GIScience approach. Based on these 11 papers and the many abstracts archived in a series of three planetary dunes workshops at the Lunar Planetary Institute (http://www.lpi.usra.edu/publications/abstracts.shtml; LPI Contributions 1403, 1552, and 1673) it is evident that there exists diverse and complementary approaches to the comparative study of landforms and processes for terrestrial and extra‐terrestrial aeolian landscapes that is driven by a vibrant international community of scientists and educators that engage in the cross‐fertilization of ideas. Copyright © 2014 John Wiley &amp; Sons, Ltd.