Eolian Systems
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This document summarizes eolian (wind-related) depositional environments and processes. It describes how deserts are commonly found between latitudes 10-20 degrees north and south of the equator. Mountain ranges can create rain shadows where dry air descends on the lee side, leading to arid conditions. Wind is the dominant agent in deserts, sorting sediments by size as it blows - with clay and silt carried high into the atmosphere as loess while sand is deposited in dunes. Dunes come in different shapes depending on variables like sand supply and wind direction/consistency.
Eolian Systems
- 2. Locations of Major Deserts Deserts are found primarily in latitudinal belts between 100 and 200 north and south of the equator.
- 3. Changing Environments Climatic conditions can vary between humid and arid at a single location because of actual fluctuations in the climate or by migration of plates through different climatic belts.
- 5. Photo by W. W. Little
- 6. Rain Shadows Mountain ranges can create barriers that force humid air higher into the atmosphere where it condenses and falls as precipitation. Dry air then descends on the opposite side absorbing moisture from the air and surface.
- 7. Coastal Dune System Dune fields are common features in sandy coastal regions, where fine- grained material is blown landward from beaches.
- 8. Photo by Ian & Tonya WestPilot Point (Shell Bay)
- 9. Simplified Model Eolian deposits tend to be relatively uniform in character, consisting of a single major facies association.
- 10. Eolian Processes Wind is the primary hydrologic agent operating in desert regions. The low viscosity of wind makes it a highly effective sorting agent. Particles that cannot be transported, such as gravel and course sand are left behind as desert pavement. Medium and fine sand are piled into dunes. Silt and clay are blown high into the atmosphere and settle elsewhere as loess.
- 11. Dune Processes Dunes are the primary depositional feature related to eolian processes and are formed as sand grains migrate up the stoss side of the macroform and avalanche down the lee face.
- 12. Saint Anthony Sand Dunes at Egin Lakes
- 13. Dune Origin Sand will often begin to accumulate behind a barrier, such as a fence post, a rock, or a bush, that creates a wind shadow.
- 19. Dune Growth Once a small pile of sand has accumulated, it begins to move as single mass, called a dune. Small dunes coalesce into larger dunes.
- 21. Dune Fields Because small dunes migrate more rapidly than large dunes, they eventually combine to form a larger body called a dune field, which migrates very slowly relative to the smaller features.
- 23. Sand Seas (draas) Very large sand bodies, such as that which covers the Sahara Desert, are referred to as sand seas.
- 24. Dune Field Margins The margins of dunes and dune fields are often very sharp and distinct.
- 26. Dune Migration Dunes are asymmetrical features. Sand is eroded from and blown upward along the gentle side of the dune, where It piles up at the crest before avalanching down the steep “slip face.” This forms a diagonal pattern referred to as cross-bedding.
- 27. Flow Separation Already well-sorted sediment becomes even better sorted at the dune crest, where finer grains are suspended by momentum and settle onto the back of a leading dune; whereas, coarser particles avalanche down the dune front.
- 28. Eolian cross-bed sets can be exceptionally thick, potentially exceeding 15 m.
- 29. Common Eolian Facies Eolian deposits are dominated by very fine-grained, trough or planar (depending upon dune morphology) cross-bedded sand that is both compositionally- and texturally-mature. Thin beds of parallel-laminated sand can separate major cross-bed sets. Occasional interdune lacustrine deposits can also be found. In some areas, alluvial fan, braided stream, and playa deposits can be found in association with eolian deposits.
- 30. Planar Cross-stratified Sand Planar cross-stratified sand forms by migration of straight-crested (transverse) dunes.
- 33. Straight-crested (transverse) Dunes Transverse dunes are elongate ridges with relatively-straight fronts oriented perpendicular to wind direction. These dunes form under conditions of an abundant sand supply and a consistent wind direction.
- 35. Trough Cross-stratified Sand Trough cross-stratified sand forms by migration of sinuous-crested (barchan, barchanoid, & parabolic) dunes.
- 37. Sinuous-crested (barchanoid) Dunes Barchanoid dunes are elongate ridges with curved fronts oriented perpendicular to wind direction. These dunes form under conditions of a moderate sand supply and a consistent wind direction. They are a transitional form between barchan and transverse dunes.
- 39. Barchan Dunes Barchan dunes are crescent-shaped with the horns pointing down wind. These dunes form under conditions of a limited sand supply and a consistent wind direction.
- 42. Parabolic Dunes Parabolic dunes are crescent-shaped with the horns pointing up wind. These dunes form under conditions of a limited sand supply and a consistent wind direction. The horns are reversed from barchan dunes because of anchoring by vegetation.
- 43. Dune Morphology There are several common shapes for sand dunes. These shapes are controlled by sand abundance, wind velocity and duration, consistency of wind direction, and presence of vegetation.
- 44. Longitudinal Dunes Longitudinal dunes are narrow, elongate ridges oriented parallel to wind direction. These dunes form under conditions of a limited sand supply and at least two prominent, sub-parallel wind directions.
- 47. Star Dunes Star dunes form central peaks with three or more arms that radiate outward. These dunes form under conditions of an abundant sand supply and a variable wind direction.
- 51. Ripple-bedded Sand The most common feature on the surface of sand dunes in ripple marks. These are essentially small-scale dunes that form from gentler winds between major storms.
- 56. Avalanche Structures Sand on the dune slip face accumulates at the angle of repose, leading to common avalanching during and between storm episodes.
- 58. Contorted-bedding in Sand The most common feature on the surface of sand dunes in ripple marks. These are essentially small-scale dunes that form from gentler winds between major storms.
- 59. Tracks & Trails Despite the arid conditions, many plants and animals make their home on sand dunes. Because of the very fine nature of the sand, great detail can be preserved.
- 69. Rain Imprints
- 70. Interdune Ponds During the wet part of climatic cycles, ponds often form in the troughs between dunes. Vegetation begins to grow and helps to, at least temporarily, stabilize the dunes and prevent their migration.
- 73. Blowouts Blowing wind may form erosional depressions called blowouts that can later be filled, forming lens-shaped deposits.
- 74. Large-scale Architecture In areas of abundant sand and high winds dune trains can form, where one dune climbs up the back of another. The result is a pattern of cross-bed sets separated by erosional surfaces.
- 82. Loess Silt and clay are picked up and blown high into the atmosphere. It settles far from the source, producing thick accumulations called loess.
- 83. Photo by W. W. Little
- 84. Photo by W. W. Little
- 86. The Dust Bowl