The transport of sand and dust by wind is a potent erosional force, creates sand dunes and ripples, and loads the atmosphere with suspended dust aerosols This article presents an extensive review of the physics of wind-blown sand and dust on Earth and Mars Specifically, we review the physics of aeolian saltation, the formation and development of sand dunes and ripples, the physics of dust aerosol emission, the weather phenomena that trigger dust storms, and the lifting of dust by dust devils and other small-scale vortices We also discuss the physics of wind-blown sand and dune formation on Venus and Titan

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The physics of wind-blown sand and dust

The transport of sand and dust by wind is a potent erosional force, creates sand dunes and ripples, and loads the atmosphere with suspended dust aerosols. This paper presents an extensive review of the physics of wind-blown sand and dust on Earth and Mars. Specifically, we review the physics of aeolian saltation, the formation and development of sand dunes and ripples, the physics of dust aerosol emission, the weather phenomena that trigger dust storms, and the lifting of dust by dust devils and other small-scale vortices. We also discuss the physics of wind-blown sand and dune formation on Venus and Titan.

/pdf/the-physics-of-wind-blown-sand-and-dust-4ggy9vm6xy.pdf

Carbonate ramp depositional systems

Sedimentary rocks at Eagle crater in Meridiani Planum are composed of fine-grained siliciclastic materials derived from weathering of basaltic rocks, sulfate minerals (including magnesium sulfate and jarosite) that constitute several tens of percent of the rock by weight, and hematite. Cross-stratification observed in rock outcrops indicates eolian and aqueous transport. Diagenetic features include hematite-rich concretions and crystal-mold vugs. We interpret the rocks to be a mixture of chemical and siliciclastic sediments with a complex diagenetic history. The environmental conditions that they record include episodic inundation by shallow surface water, evaporation, and desiccation. The geologic record at Meridiani Planum suggests that conditions were suitable for biological activity for a period of time in martian history.

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In situ evidence for an ancient aqueous environment at Meridiani Planum, Mars.

Bagnold, R.A. 1941: The physics of blown sand and desert dunes. London: Methuen:

The thinnest recognizable strata in modern eolian dune sands can be grouped into six classes. They are herein named planebed laminae, rippleform laminae, ripple-foreset crosslaminae, climbing translatent strata, grainfall laminae, and sandflow cross-strata.

Planebed laminae are formed by tractional deposition on smooth surfaces at high wind velocities. They are very rare in the deposits studied. Grainfall laminae are also formed on smooth surfaces, largely by grainfall deposition in zones of flow separation. They are much more common than planebed laminae, which they closely resemble.

Eolian climbing-ripple structures are composed primarily of climbing trans-latent strata, each of which is the depositional product of a single climbing ripple. Climbing translatent strata that formed at relatively high or supercritical angles of ripple climb are typically accompanied by rippleform laminae, which are wavy layers parallel to the rippled depositional surfaces. Ripple-foreset crosslaminae, which are incomplete rippleform laminae produced when the angle of ripple climb is relatively low or subcritical, are rarely visible in eolian sands.

Sandflow cross-strata are formed by the avalanching of noncohesive sand on dune slipfaces. Their form varies with slipface height and with other factors.

Basic types of stratification in small eolian dunes

The marginal marine environment consists of an offshore where the wave form is approximately sinusoidal and a nearshore where the wave form is either solitary or that of a bore. Within the nearshore, shoaling waves become progressively higher and steeper until they break. After breaking, the waves progress as bores through a surf zone; these bores ultimately terminate within a swash zone on the beach itself. In a high-energy coastal environment where long-period swell enters a nearshore uncomplicated by offshore bars, sedimentary structures develop on the seafloor in facies that trend parallel to the zones of different wave activity. In the offshore, small sand ripples are the most common depositional structure, but in the nearshore larger bed forms predominate. Seaward from the line of breakers, in the zone of wave build-up, are landward-oriented lunate megaripples. Near the outer portion of the surf zone the bed form is planar (outer planar facies), but, in the inner portion of the zone, a area of large-scale bed roughness (inner rough facies) commonly is present. Within the swash zone, the bed form is again planar (inner planar facies). The boundaries of the facies shift in response to changes in waves or tide, and certain of the zones are sometimes missing. The relative position of the zones, however, is invariable. The major features of the bed forms can be interpreted in terms of flow regime. The stronger of the two opposing transient currents caused by passing waves produces structures analogous to those produced by continuous, unidirectional currents. Landward wave surge is dominant in the outer three structural facies, whereas seaward surge predominates in the innermost (inner planar) facies. Wave surge over the intermediate inner rough facies is more complex, and the direction of strongest surge may be variable. In the outer three facies, the landward sequence from small asymmetric ripples to lunate megaripples to plane bed suggests an increase in flow regime from the lower part of the lower regime to the upper regime; this shoreward increase in flow regime is associated with a shoreward increase in orbital velocity at the bottom. The inner planar facies is produced by flow in the upper regime. The inner rough facies, situated between two zones of flow in the upper regime, is apparently a product of flow in the upper part of the lower regime. Within each of the structural facies a distinctive set of internal structures is produced. Internal structure of the asymmetric ripple facies consists of shoreward-inclined ripple cross-lamination and gently inclined cross-stratification. The lunate megaripples produce medium-scale landward-dipping foresets. Within the outer planar facies bedding is nearly horizontal. Structures in the inner rough facies produce medium-scale foresets that mostly dip directly or obliquely seaward, although landward-dipping foresets also occur. Within the inner planar facies bedding is gently inclined seaward. Migration of the facies in response to changes in waves or tide produces distinctive assemblages of structures where the facies overlap. These assemblages provide criteria for paleoenvironmental i terpretation, particularly where interrelated assemblages occur in a meaningful spatial distribution.

Depositional Structures and Processes in the Non-barred High-energy Nearshore

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

http://science.sciencemag.org/content/sci/237/4812/276.full.pdf

Bedform Alignment in Directionally Varying Flows

Where bedforms migiate during deposition, they move upward (climb) with respect to the generalized sediment surface. Sediment deposited on each lee slope and not eroded during the passage of a following trough is left behind as a cross-stratified bed. Because sediment is thus transferred from bedforms to underlying strata, bedforms must decrease in cross-sectional area or in number, or both, unless sediment lost from bedforms during deposition is replaced with sediment transported from outside the depositional area. Where sediment is transported solely by downcurrent migration of twodimensional bedforms, the mean thickness of cross-stratified beds is equal to the decrease in bedform cross-sectional area divided by the migration distance over which that size decrease occurs; where bedforms migrate more than one spacing while depositing cross-strata, bed thickness is only a fraction of bedform height. Equations that describe this depositional process explain the downcurrent decrease in size of tidal sand waves in St Andrew Bay, Florida, and the downwind decrease in size of transverse aeolian dunes on the Oregon coast. Using the same concepts, dunes that deposited the Navajo, De Chelly, and Entrada Sandstones are calculated to have had mean heights between several tens and several hundreds of metres.

Bedform climbing in theory and nature

Many of the straighter, less rocky parts of the southern Oregon coast are characterized by nearshore bars that extend obliquely out from shore and migrate alongshore. A typical oblique bar is attached to the foreshore at its upcurrent end, nearly parallel to shore through most of its length, and bowed seaward into a rip-channel-mouth bar at its downcurrent end. The main part of an oblique bar is separated from the foreshore by a longshore trough that curves seaward into a rip channel. The net wave-induced currents flow obliquely shoreward over the bar, parallel to shore through the longshore trough, and seaward through the rip channel and over the rip-channel-mouth bar. Medium-scale crossbedding formed by the migration of megaripples in the direction of net water flow is the dominant nternal structure in most of the nearshore. Small-scale crossbedding formed by wave-ripple migration is dominant in the inner offshore, and planar bedding is dominant on the foreshore. Most or all of the bar deposits would be destroyed if the coast prograded slowly but continuously. The vertical sequence produced during progradation should be characterized, in ascending order, by inner offshore deposits, possibly deposits of the lower seaward slope of the bar, a subhorizontal erosion surface corresponding to the deepest part of the rip channel, rip channel deposits, longshore trough deposits, deposits of the foreshore-trough transition, and foreshore deposits, Subhorizontal erosion surfaces probably occur in deposits formed by the slow progradation of most kinds of barred nearshore systems, but they should not occur in deposits formed by the progradation of a non-barred nearshore system. The nature of the deposits produced by the progradation of a barred nearshore system should vary with orientation of the bars relative to the shoreline, number of bars, wave energy, and available grain sizes.

Ralph E. Hunter

Papers

Basic types of stratification in small eolian dunes

Depositional Structures and Processes in the Non-barred High-energy Nearshore

Bedform Alignment in Directionally Varying Flows

Bedform climbing in theory and nature

Depositional Processes, Sedimentary Structures, and Predicted Vertical Sequences in Barred Nearshore Systems, Southern Oregon Coast