Common aeolian bedforms

A series introducing aeolian processes and deposition, comparing the conditions on Earth and Mars.

Key words: sand dunes, ripples, crossbeds, density ratios, saltation, impact creep, reptation, ripple laminae, airfall, deflation, grain flow, avalanche.

The characteristic modes of sediment transport in water and air are fundamentally different. In water the dominant mode is traction, where a carpet of granular material is moved by rolling, sliding, and the occasional bounce. In contrast, saltation is the dominant mode of sand transport in aeolian systems. The different modes of transport are primarily a function of the difference in density ratios between the solid phase and fluid phase; (on Earth) the density ratio in air is about 2000 times greater than that in water. This means that sand grains in air have greater inertia such that greater velocities are required to overcome drag forces and set grains in motion – sand grains also have greater momentum once they are in motion. Saltation also occurs in water borne sediment movement, but the mechanism in air typically produces longer and higher trajectories.

Saltation is the dominant mode of sand transport in aeolian systems, but three other mechanisms also contribute to sediment movement:

Impact creep where airborne grains land and mobilize other grains along the bed.
Reptation where the splash effects of landed grains launch other grains in short saltation hops.
Suspension where very small particles are held aloft for much longer periods of time than the saltation load.

The different mechanisms are nicely illustrated by the formation of current ripples and their corresponding stratification. In subaqueous ripples, sand is moved to the brink of the stoss face whereupon it tumbles, rolls, or avalanches down the lee face. The characteristic bedform is lee face crossbedding, or ripple crossbedding. In contrast, the characteristic and most common bedform developed from aeolian ripples is parallel lamination.

Aeolian ripples and ripple lamination

Sand dune and interdune surfaces are commonly veneered by small scale ripples. Most are straight crested, locally bifurcated, with wavelengths 20-50 cm. Aeolian ripple wavelength/amplitude ratios, also called the ripple index (RI), commonly range from 15 to 50, averaging about 18, which is a significantly greater than the indices for subaqueous sand ripples (average RI of 10 to 15). This means that aeolian stoss face inclinations are less than their watery counterparts.

[The concept of ripple index was introduced by R.P. Sharp, 1963, who observed that the numerical value of the ratio varied inversely with grain size and directly with wind velocity ]

R.A. Bagnold (1941, 1954), J.R.L. Allen (Current Ripples, 1968), and R.E. Hunter (1977) have all contributed to our understanding of ripple formation. Ripples advance downwind of their lee slopes. If there is no net sand accumulation during their advance, then there is no stratigraphic record of the event. If sand accumulates in front of the lee slope, then the ripple must climb over it in order to advance. The angle of climb dictates the kind of ripples that form and their depositional record. The angle of climb reflects the ratio between the rate of sand accumulation and the rate of ripple migration. The critical angle is the inclination of the corresponding stoss slope.

The angle of climb under subcritical conditions is too low to preserve either the stoss or lee slopes and the resulting bedform is parallel lamination. This is the most common type of small-scale bedform in the aeolian stratigraphic record. Laminae are typically <10 mm thick, and commonly exhibit inverse grain size grading that reflects the accumulation of slightly coarser material along ripple crests. Normal density grading also means that heavy minerals concentrate along the base of laminae. Both forms of grading result in subtle colour changes within laminae – the variations in colour are also referred to as pinstripe bedding. However, aeolian dune sands are typically very well sorted and grain size grading may be too subtle to differentiate; this problem also applies to identification of small-scale crossbedding.

As wind velocities increase, so too does the ripple climb angle. At supercritical angles (i.e. > the stoss slope) both stoss and lee faces are preserved. This is manifested as lee face crossbedding in sets bound by wavy or planar contacts; wavy contacts mimic ripple profiles. Wind frequently occurs in gusts across dune surfaces – the potential stratigraphic record may show the transition from subcritical parallel lamination to supercritical climbing ripples.

Airfall laminae

Flow separation downwind of a ripple or dune brink results in the loss of grain momentum in the saltation load. The grains fall to the lee face with coarser varieties near the top, and finer grains farther down the lee face. Identifiable airfall laminae tend to be a few millimetres thick. They may extend the length of the lee face or pinch out part way down depending on sediment supply. Laminae drape pre-existing structures (lee face ripples, avalanche deposits).

Airfall deposition is one of the main drivers of downwind bedform migration. They form the characteristic lee face cross stratification. Airfall deposition results in gradual steepening of the lee face until it reaches the angle of repose. Subsequent failure produces grain flow and avalanche deposits that contribute to the overall bedform architecture of ripple and dune stratification.

Airfall laminae tend to be thinner than ripple lamination and also lack grain size grading, but these differences may be too subtle to confidently differentiate the two bedforms, particularly in well-sorted sand.

Plane bed lamination

The aeolian equivalent to subaqueous plane bed deposits theoretically develop when wind velocities are too high for ripple formation. In subaqueous settings, this condition applies close to the transition from Froude subcritical (tranquil) flow to supercritical flow. These conditions are not as well documented in aeolian systems. It is also not clear how plane bed lamination can be differentiated from airfall lamination.

Large scale crossbeds

Aeolian crossbedding is one of the most spectacular bedforms in the rock record. Crossbed sets can be many metres thick, such as the iconic Early Jurassic Navajo Formation. Most dune interiors consist of multiple crossbed sets that represent multiple episodes of bedform truncation, reactivation, and superposition that reflect changing conditions of sand supply, wind direction and wind strength. Changing moisture content may also play a role in crossbed architecture, for example rainfall in coastal dunes, or dunes associated with rising watertables.

Most large-scale dune crossbeds accumulate via lee face migration. Lee face stratification includes airfall lamination, subcritical ripple lamination, and grain flow-avalanche bedforms. All four bedforms can comprise a single crossbed set.

Grain flows and avalanches

Grain flows and avalanches (also called translational slides) are a common feature of terrestrial and Martian dunes – described in the post on the Bagnold dunes, Gale crater. Both processes contribute to the advance of lee faces and to dune stratigraphic architecture. Translational slides are probably the easiest to identify in outcrop. Slides move as relatively coherent blocks and slabs of relatively cohesive sand, although they can rotate and under extension on the upper lee face can form boudinage-like geometries. Blocks lower down the lee face tend to be under compression and can be stacked structurally.

Adhesion ripples

Saltating grains that land on a wet surface commonly form irregular, patchy, discontinuous ridges a few millimetres thick. They are commonly found on beaches, and on interdune flats after rainfall, or where the watertable rises to the surface. The structures degrade rapidly once the sand dries. Adhesion ripples have low preservation potential.

Deflation lags and surfaces

Erosion and abrasion of sediment, bedrock occur in areas of limited sand supply. Abrasion by airborne saltating sand grains can modify landscapes, expose and shape in situ bedrock and larger sedimentary fragments. Coarse-grained debris is commonly concentrated across exposed surfaces where finer-grained particles have been removed by strong winds. These processes can be enhanced by sea and salt-lake spray. Particles produced by abrasion contribute to the local sediment load.

The largest representatives of this category of bedform are yardangs. Yardangs on Earth are subparallel ridges and furrows formed by wind erosion and deflation of sediment or bedrock. They are streamlined, teardrop shaped with the broader, steeper face upwind. The consistency of these geometrical attributes means they can be used to decipher wind directions. Yardangs have well defined boundaries and do not originate from obstacles, characteristics that distinguish them from wind streaks. Yardangs have also been identified on Venus in an area near Mead crater; they are about 25 km long and 0.5 km wide. (Greeley et al., 1992), and on Mars.

Coarse-grained lags are common on land surfaces where fine-grained sediment has been removed by strong winds, leaving a concentration of large clasts. Exposed clasts shaped by sand-blasting are called ventifacts. Deflation lags including ventifacts are also common on Mars.

Deflation lags have high preservation potential due largely to their grain size. In the stratigraphic record they help define subaerial stratigraphic discordances.