Aeolian systems exist as a dynamic continuum that, for convenience, are assigned dry, wet, and stabilized end members.
Aeolian dunes form wherever there is sufficient dry, cohesionless sand and winds strong enough to transport it. These conditions apply not just to terrestrial dune systems, but also to those on Mars and probably those on Venus and Titan. Dune fields, that include ergs (sand seas), coastal dunes, and sandurs, reflect the interaction between sediment transport, deposition, and accumulation. Accumulation represents the potential for a stratigraphic record that reflects the sum-total of these aeolian processes.
The primary components of dune fields are:
- the dunes themselves, ranging from small transverse bedforms to massive draas.
- Interdune areas that range from narrow troughs or depressions to broad interdune flats. Interdune flats may be devoid of loose sediment (e.g., bedrock, deflation surfaces, evaporite crusts), be covered by a veneer of sand, or contain vegetated or ponded areas that accumulate mud and soils.
- Vegetation acts as a baffle to sand movement and promotes dune and interdune stabilization and soil formation. Vegetation can also be domicile to vertebrate and invertebrate faunas.
Stratigraphic accumulation within this mix of components depends on the nature of the depositional surface. Kocurek and Havholm (1993) define a depositional surface as a surface of sediment transport, commonly manifested as bedforms. Accumulation can only occur below this surface. Note that the space for accumulation (accommodation space) does not include the bedforms that migrate across the surface. Thus, the depositional surface must rise for accumulation to take place. If accumulation does not occur then the only record of sand dunes that passed that way might be a deflation surface veneered by coarse grained, faceted rock fragments, mineralized crusts (evaporite, ferricrete, iron pans, silcrete), or erosional structures like yardangs.
The dynamics of aeolian accumulation are conveniently described by three systems – dry, wet, and stabilized, that can be thought of as end-members in a spectrum of aeolian systems ( Kocurek and Havholm, 1993 op. cit.) ; the three systems are not mutually exclusive.
Dry dune systems
Dry dune systems occur where the watertable and its capillary fringe are too deep to influence sand transport and deposition. Sands in this case are basically cohesionless. Sand carried up a dune stoss face by flow acceleration is deposited on the corresponding slip, or lee face in a zone of flow separation and deceleration. Flow is reattached on the next downwind stoss face. In this way the dune migrates across the intervening interdune area. It is generally observed that sand entering the interdune areas is mostly returned to the active dunes; i.e., they are areas of sediment bypass.
Although flow deceleration is a well-established process over individual dunes and interdune flats, it also occurs more generally over the downwind length of an entire dune field (Kocurek, 1988). Wind deceleration causes a decrease in dune spacing at the expense of interdune areas; dune size tends to remain constant. Interdune areas are reduced from broad flats to narrow troughs; the dune bedforms are now close enough that they begin to climb. The angle of climb is the ratio of the rate of dune migration and rate of accumulation. This is the stage in dry dune field development where accumulation can occur because the depositional surface (defined by the depth of the interdune troughs) overlaps the lee-face stratification of the preceding dune. Some erosion of lee face deposits may occur as dunes advance; sand released in this way will be added to succeeding bedforms.
As a first approximation, interdune deposits will be absent or poorly represented in dry dune systems. However, variations in stratigraphic architecture can be expected because wind direction and dune migration can vary locally, or when wet conditions occur periodically. A classic example is the Navajo Sandstone, beautifully exposed at Zion National Park, that Rubin and Hunter (1982) interpret as a dry aeolian system. Here, the base of dune foresets are commonly tangential to the lower bounding surface and interpreted as basal aprons or plinths of lee faces that terminated in narrow interdune troughs rather than broad interdune flats.
Wet dune systems
Dune fields in which the depositional surface interacts directly with the local watertable or the capillary fringe (above the watertable) are referred to as wet systems. The cohesiveness of wet sand prevents bedload transport. Thus, stratigraphic accumulation of sediment in this system ‘depends on changes in the watertable elevation. If the watertable and its capillary fringe are lowered, sand will be removed from interdune flats until a new depositional surface is established (the sand will be moved to active bedforms). A rise in the watertable will provide additional accommodation space for sediment to accumulate.
Short term changes in watertable elevations occur because of seasonal and multiyear variations in groundwater recharge; the watertable in many aquifers can vary by more than a metre. Diurnal changes in watertable elevation can also occur in coastal dunes where groundwater is hydraulically connected to ocean tides.
Longer term changes in watertable elevations are mainly controlled by climate or basin subsidence. Trends towards wetter or drier climates will elevate or lower regional watertables relative to some baseline. Basin subsidence will always increase accommodation space where the depositional surface is lowered but the watertable remains at the same elevation. These two drivers of watertable change are often considered separately, but in reality both are likely to occur simultaneously. In this case, long term subsidence will enhance the available accommodation caused by increased aquifer recharge.
With a rising watertable, accumulation will occur if sediment supply is positive such that more sediment is stored in dunes and interdune areas than bypasses the system. If sediment supply is relatively low (but still positive) then interdune flats will tend to expand at the expense of sand dunes. If sediment supply increases, then dune size will increase; if supply greatly exceeds storage, the area occupied by active dunes may increase to the point where the system becomes ‘dry’. The preservation of both interdune and dune deposits also requires a climbing depositional surface. If the angle of climb is zero, then sediment bypasses the system and a surface of discontinuity forms – a supersurface will form if sediment bypass encompasses a large part of the dune system (erg, coastal dunes).
An idealized stratigraphic succession developed during a rising watertable is shown in the following numerical model (modified from Bristow and Mountney, 2013, Fig 13).
The stratigraphic succession generated in the model is relatively simple where dune amplitude and interdune spacing remain constant, and sand supply, accumulation rate, and climb angle are also constant. The accumulation rate is directly linked to the uniformly rising watertable. The angle of bedform climb and interdune spacing dictate the thickness of preserved dune crossbedding. The succession in this case is a gently dipping alternation of dune and interdune deposits. Although this is an ideal, simple case, it illustrates the general principles that promote accumulation in wet conditions.
Complications to this simple model will arise because sediment transport, and accumulation or bypass typically vary depending on variable local topography, wind strength and direction, sand supply, dune orientation, and the presence of vegetation. For example, if dune wavelength and interdune size increase downwind, then the resulting thickness of accumulated dune bedding will decrease because the angle of climb decreases. A downwind decrease in the thickness of preserved dune bedding will be compensated to some extent by an increase in thickness of interdune deposits. A wet aeolian system can evolve to the point where interdune deposits are significantly thicker than the dune deposits.
Stabilized dune systems
Dunes and interdune areas can be stabilized by vegetation (the most common stabilizing factor), or deflation lags, cements and evaporite crusts, and mud veneers. Except for lag deposits, most other stabilizing agents involve water or moisture and hence stabilized aeolian systems are commonly thought of as wet systems. However, they can also bridge the gap between wet and dry conditions. Stabilizing agents operate continuously or discontinuously.
Stabilizing vegetation is particularly common in coastal dune complexes. The vegetation acts as a baffle that causes deceleration of wind and accumulation of sand. Persistent vegetation cover in interdune areas can eventually lead to soil development. However, soil development over sand dunes is commonly interrupted by blowouts that remobilize the sand. Plant roots can extend more than 2 metres into dune sands to access moisture.
Stabilized coastal systems are likely to be wet close to shore because watertables in those locations are hydraulically connected to sea level and therefore tend to be very shallow. However, farther inland, for example on a coastal plain, the watertable will tend to deeper and the dune system may transition to dry conditions.
The dune continuum
Dry and wet dune systems are end members of a continuum that respond to variations in sediment supply, variable wind strengths and directions, and the degree of substrate ‘wetness’. The continuum reflects spatial and temporal changes:
- Spatial in the context of variations within a single dune field where, for example, some parts of the field are subjected to watertable influences and others are not. This is likely to be the case in many coastal dune systems where watertable aquifers are hydraulically connected to sea level. Close to shore, the watertable or capillary fringe will be close to the depositional surface but will occur at greater depths below the surface farther inland. A modern example where dry, wet and stabilized dunes occur in the same dune field is the Skeiđarársandur, a glacial outwash sandur along the southeast coast of Iceland (Mountney and Russell, 2009). Sediment availability and dispersal in the Skeiđarársandur system is strongly influenced by seasonal freeze, ice sublimation, and thaw.
- The dune system as a temporal continuum records stratigraphic changes driven by autogenic controls within the system (e.g., development of stabilizing flora), and allogenic controls driven by changing climate, external sources of sediment, and base level (e.g., subsidence, glacioeustacy). The stratigraphic record may also include supersurfaces that represent the termination of dune field activity because of sediment bypass or erosion.
Other posts on aeolian systems
Bounding surfaces in aeolian dunes
Wind-blown sand on Mars, Venus, and Titan
Evidence for bedload deposition on Mars
Atmospheric circulation on Mars: An aeolian context
