Aeolian Dunes and Sandstone: Overview
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The following is informed by definitions from the Glossary of Geology produced by the American Geological Institute (recommended to authors by the Geological Survey of Canada), and by usage in recent literature. Many of these terms have synonyms, and are also used in other contexts.

Aeolian (also spelled eolian) means 'wind-blown'; Aeolus was ruler of the winds in Greek mythology. Aeolian transverse and barchan dune types have an idealized cross-section as shown in figure 1 below, with a slipface on their lee (downwind) side.

Fig 1. Cross-section of a transverse dune

The acute angle of the slipface with respect to horizontal (as measured in a vertical plane normal to the slipface) is the dip; the dip of the slipface in figure 1 is about 40 degrees. The dip direction is the compass direction toward which the slope faces (eg., the direction toward which water would flow, if the slope were impermeable). A line of strike is a line formed by the intersection of the plane of the dipping surface (eg., the slipface) and a horizontal plane; in the case of a dune, the brink is a line of strike. (Lines of strike are perpendicular to the direction of dip.)

The slope angle at which sand begins to avalanche is the angle of yield. The angle at which avalanching sand comes to rest is the angle of repose. These angles depend on characteristics of the sand such as grain sizes, grain shapes, and moisture content. Dry aeolian dune sand typically has an angle of repose of about 34 degrees. The angle of yield is usually a few degrees greater than the angle of repose.

Transverse dunes migrate, as illustrated in figure 2, when strong winds pick up sand grains from the stoss slope of the dune (upwind or windward side) and blow them across the crest and brink, where they fall onto the slipface. Protected from the wind, they accumulate on the upper slipface to a low, broad mound or cornice1. Eventually the cornice deposit grows too steep (its lower slope exceeds the angle of yield) and its sand begins to flow as an avalanche down the slipface, shifting the dune downwind.

All other factors being equal, a dune's advance depends on its size, because it can only advance by adding a layer of sand to its slipface -- the longer (higher) the slipface, the larger the volume of sand required per unit of advance. Travelling at a speed (celerity) of tens of meters per year would be a lot of travel for a dune many meters high.

Fig 2. Dune migration by avalanching (press Esc to stop animation, PageRefresh to resume)

If erosion of the stoss surface is less than that of a preceding dune, as in figure 2, a migrating dune leaves a layer behind consisting of what were the lower parts of the dune (left side of figure 2). A subsequent dune may later travel over this left-over layer, also leaving a layer consisting of its lower parts; in this manner, sand can accumulate in layers and become preserved. Figure 3 below shows three such layers (and the partially eroded remains of a fourth, at the top) in Navajo Sandstone (Utah, USA). These layers are many meters thick (there is a person sitting at the edge of the shadow at the lower-right).

Fig 3. Navajo Sandstone, Utah, USA

Seasonal Cycles of Deposition

The illustration below shows the idealized effect on a dune of strong winter wind alternating with weaker reverse-direction summer wind. The weaker summer winds in this example cannot completely reverse the migration that occurred during the winter, but do alter the structure of the dune and leave a wedge-shaped deposit at the toe of the slipface, where they are often preserved.

Fig 4. Seasonal cycles of deposition (press Esc to stop animation, PageRefresh to resume)

Descriptive Terminology

Descriptive terminology gives a vocabulary for talking about sandstone outcrops without interpretation.

A cross-stratum (plural: cross-strata) is a layer or bed that is inclined relative to a larger context (eg., the floor of a dune). A set of cross-strata is the entire series of adjacent cross-strata between two surfaces. Two or more adjacent sets are called cosets ('co-sets'). Thus the bulk of the outcrop in figure 3 consists of three sets of cross-strata.

Fig 5. Cross-strata, sets, and cosets

A stratum less than 1cm thick is a lamina; however, the term 'lamina' is often used as a synonym of stratum. The plural of lamina is laminae.

A pinstripe (or 'pin stripe') is a thin lamina, in the range of 1mm thick.

The cross-strata in figure 5 are said to downlap, because they are inclined layers terminating against a less inclined surface.

Two shapes are commonly distinguished when viewed in cross-section (perpendicular to their plane): tabular (rectangular) or wedge (triangular) -- or, referring to their shape in three dimensions, tabular-planar or wedge-planar.

The surface dividing sets (visible as a curve in cross-sectional view) is a bounding surface. Thus figure 5 above has three sets of cross-strata; the upper three-quarters of each set's cross-strata is tabular, and the lower one-quarter of each set has a gradually decreasing dip, downlapping asymptotically to a horizontal bounding surface; the cross-strata are concave-up (have a curve with its inside, the concave part, facing up).

Interpretative Terminology

Interpretative terminology gives a vocabulary that communicates the results of interpretation and recognition. Interpretation attempts to add value to the bare facts: 'bounding surface' may become after interpretation 'interdune migration surface'.

Aeolian sandstones are those interpreted as being lithified remains of aeolian sand dunes. Dune terminology can be loosely grouped into 'geomorphic' and 'genetic'.

Fig 6. Geomorphic terms

Geomorphic dune terminology is based on the external shape and form of a sand dune. The lee side (or leeward slope) is downwind of the crest, and the windward side or stoss slope is upwind (windward) of the crest. The brink divides the top of the dune from the inclined slipface. Sand blowing over the brink falls into a wind shadow and accumulates in a cornice,1 a low mound on the upper slope of the slipface. Eventually the cornice grows too large and its sand avalanches down the slipface, leaving a relatively flat surface inclined at the angle of repose. At other times, reverse winds or cross-winds can cause sand to accumulate at the base of the dune, creating an apron. The point between the curved (concave-up) apron surface and the slipface is the toe. The area between dunes, generally flat and often erosion-resistant, is the interdune.

Fig 7. Genetic terms

Genetic terminology emphasizes how structures formed. Grainflow (or 'sandflow') layers are created by sand grains flowing down a slope. Grainfall deposits are created by sand grains dropping more or less ballistically from the air (such as occurs to form a cornice at the top of the slipface, prior to avalanching). Wind-ripple laminae are left by migrating dry-sand ripples.

Bounding surfaces are surfaces separating distinct sets of strata. Bounding surfaces are known to be created by (at least) the following mechanisms (referring to figure 8 below):

  • An interdune migration surface (or first-order bounding surface) is the interdune surface (left behind by stoss-side erosion of a passing dune, and possibly further eroded as an interdune surface). Moist sand is more resistant to erosion than dry sand, so sometimes the water table controls the extent of interdune erosion. Interdune migration surfaces tend to be near horizontal.
  • A superimposition surface (or second-order bounding surface) is created by the migration of one dune (usually a secondary smaller dune) over another dune. This can happen without erosion of the underlying dune.
  • A reactivation surface (or third-order bounding surface) is created when a deposition process is temporarily interrupted by a change of wind direction causing erosion. The erosion surface becomes a reactivation surface if later deposition resumes in the original direction, laying down more deposits in the original orientation.

Fig 8. Bounding surface types

Fig 9. Fluvial terms

Fluvial bedform terminology, developed to describe the internal structure of subaqueous ('in water') river delta deposits, is sometimes borrowed to describe similar structures of subaerial ('in air') dunes (it was only in 1977 that geologists noticed ways to reliably differentiate fluvial and aeolian sandstones; fluvial terminology is a hold-over and should probably be avoided when discussing aeolian features). River deltas grow by sediments flowing to the edge of a delta and then avalanching down into deeper water -- a process similar to wind blowing sand over a brink. Layers left by material flowing along the top (of the delta, or of the dune) are topset strata (or 'beds'). Layers left by material avalanching down the slipface, at the angle of repose, are foreset strata (or beds). Material caught at in the corner between the slipface and the interdune area is bottomset strata. A foreset (or topset, etc) is a three-dimensional object (with a shape that is generally tabular-planar), whereas a slipface is a surface (two-dimensional).

Wind-Ripple Deposits

A flat sand surface is not stable with wind blowing over it; the sand will develop ripples, or wind-ripples (prefixed with 'wind' to explicitly distinguish them from subaqueous ripples formed by water flowing over sand, which have superficially similar appearance but differ in detail).

Wind-ripples seem at least superficially like small-scale dunes; they have a similar shape, and may migrate in a similar way, by erosion from their stoss side and deposition on their lee side. If erosion on the stoss side is not complete, the each ripple will travel upon the remains of the preceding ripple's base, and there will be net deposition.

Grains of different size tend to be deposited in different parts of a ripple. This sorting leads to textual laminations in the deposits created by wind-ripples. To see why, consider the three climbing ripples in figure 10 below, where colour variations have been introduced in the ripple surface make it easier to track which part of the ripple remains after erosion of the stoss side of each ripple:

Fig 10. Wind-ripples leaving a deposit. Press Esc to
stop animation, PageRefresh to resume

Notice that at this particular angle of climb, some parts of the ripples are not preserved (the red and purple, corresponding to the stoss slope and crest).

The animation moves in discrete jumps, leaving feathery artifacts, but in nature, the ripples erode grain-by-grain, usually leaving no trace of the former surfaces -- just a smooth lamination as illustrated in figure 11 below:

Fig 11. Strata (apparent) in wind-ripple deposit
(inset shows originating ripple and climb angle)

Need a photo of planar wind-ripple laminae eroded in cross-section

Fig 12. Wind-ripple deposit

The animation above exaggerates the vertical climb; in nature, the angle of climb is usually quite shallow. Figure 12 shows the millimeter-scale layers of a sandstone wind-ripple deposit.

The synthetic laminae left by wind-ripples are called climbing translatent strata ('translatent' is a reference to the geometric operation of 'translation', reflecting that the deposits have the appearance of having been created by the translation of the ripple surface, as in the animation of figure 10). The bounding surface of each lamina is the erosion surface upon which the ripple climbed (the blue line, in figure 11). Each bounding surface is a first-order bounding surface ('interdune migration surface').

The layers in wind-ripple deposits were formed by climbing ripples, so the angle of climb determines the angle of the layers. This means the dip (with respect to the surface hosting the ripples, which maybe be inclined) will be modest, since ripples can only climb as fast as net deposition permits. In contrast, grainflow layers are always near the angle of repose (before compaction).

Wind-ripple laminations, if planar, cannot be thicker than the height of the ripples that formed them, which means they are generally less than about 0.5cm thick. In contrast, grainflow layers are generally more than 0.5cm thick (the angle of repose is higher for layers less than ~10 grain diameters, so flows are generally thicker than 10 diameters ~= 0.5cm).

Wind-ripple laminations dip (usually at shallow angle) toward the wind (upwind), whereas grainflow strata dip (usually 25 to 35 degrees) with the wind (downwind).

A layer of grainflow sand (avalanched sand) becomes a grainflow cross-stratum, but a 'layer' (surface) of wind-ripple does not become a wind-ripple stratum. Each of what look like layers in figures 11 and 12 are translatent strata (sometimes called pseudo-layers), made up of contributions from many successive wind-ripple surfaces. For example, a green stratum in figure 11 is made up of grains from the troughs of successive surfaces throughout the animation in figure 10. The discontinuous black line in figure 13 below shows contributions to the final deposit of a single, isochronous wind-ripple surface:

Fig 13. Black line fragments mark the contribution of a
single ripple surface moment to the final deposit

The ripple trace is not continuous in figure 13 because in this case the purple and red portions were eroded by subsequent ripple development, but if the deposition rate were high enough, entire isochronous surfaces could be preserved.

Particle Transport

A key mode of sand transport by wind is saltation (from Latin saltere, to dance, jump, leap), where grains lifted into the airstream are blown and bounced downwind in low, arcing trajectories. A saltating grain falling back to the surface may strike and dislodge other grains in a 'splash'.

Grains rolling along the surface under the pressure of the wind or impacts from saltating grains, perhaps smaller, undergo creep or reptation (from Latin reptare, to creep).

Smaller grains may go into suspension and be scattered as dust if they are light enough to be carried by turbulent air.


I'd appreciate suggestions: Comments

Jim Elder
2006 Dec

Notes

  1. 'Cornice' in this context is not in the Glossary of Geology but I think it'd be a good addition. Here's my thinking: A cornice is a deposit in the wind shadow (recirculation eddy) of a ridge or crest. The shape of a cornice deposit depends on characteristics of the wind and the wind-blown material -- non-cohesive materials like dry granular snow or dry sand form a low mound (the stronger the wind, the further from the brink), and cohesive materials like wet snow can form a ledge or even overhang, shaped by the eddy-current flow. On the slipface of a dry sand dune, a cornice will build until eventually its lower slope exceeds the critical angle of yield and then its sand will avalanche in a tongue of grainflow, fed by a receding scarp that eats through the cornice toward the brink.