23.4 Depositional Features of Deserts

Perhaps one of the most notable depositional features that most people tend to associate with deserts are sand dunes. In general, sand dunes form when wind-blown sand accumulates in an area due to wind speed slowing down (Figure 23.4.1). This is usually caused by a change in topography or if the sand gets caught by an obstruction, such as a rock or a plant. As dunes grow vertically, one side of the dune is cut off from the exposure of the prevailing winds. Because of this, continuous movement on that side stops and the dune becomes steeper. When enough sand accumulates such that the angle of repose is exceeded, a slip face develops where sand slides down on that side only. As dunes can migrate, former slip faces can be preserved within the dune as cross bedding, whereas ripple marks form on the surface of the dune (review sedimentary structures from chapter 6.4 as needed). In total, there are 5 specific types of dunes discussed below.

Figure 23.4.1. A simplified diagram of how sand migrates and piles up to progressively form a sand dune. Note how the slip face is the steeper side of the dune.

Barchan Dunes – forms where there is a relative small supply of sand, wind dominantly blows in one direction, and no vegetative obstacles are present. They are shaped like crescent moons. (Figure 23.4.2a – top left)

Star Dunes – develop when a dune forms then winds change direction drastically. They are characterized by a central point and 3-4 outward radiating “arms.” (Figure 23.4.2b – top right)

Longitudinal Dunes – Like with barchan dunes, these have a limited sand supply. However, unlike barchans, winds here are strong and shift back and forth. Due to the shifting winds, the slopes on both sides of the dune are roughly equal (they are both slip faces). (Figure 23.4.2c – center left)

Transverse Dunes – As with barchan dunes, the wind dominantly blows in a single direction. However, the sand supply here is plentiful. In this case, barchan dunes will form a continuous chain where they merge into transverse dunes. (Figure 23.4.2d – center right)

Parabolic Dunes – If a transverse dune has a strong wind blowout in the middle of it, the displaced sand forms a curving dune that progresses downwind. The two endpoints of the curve remain intact by the vegetation obstructions that are left. Think like a parabolic curve (a parabola) in math! (Figure 23.4.2e – bottom)

In revisiting the discussion on evaporite rocks from the sedimentary rocks chapter (chapter 6), hot deserts are characterized by high rates of evaporation. Since surface water in deserts generally lack permanent streams, outflow is rare and evaporation is the dominant process of water loss. Hence, any dissolved (or carried) salts in surface water tends to accumulate.

If evaporation occurs and copious quantities of evaporitic salts are deposited, the exposed dry, flat area is known as a playa (Figure 23.4.3a). These temporary lakes form after a large flood or a particularly wet season. If there is sufficient inflow into a low-laying area of a desert (a basin), and there’s no outflow, a salt lake forms (Figure 23.4.3b). The inflowing water, containing trace amounts of dissolved ions, is fresh enough to drink. However, the accumulated water (and salts) in a salt lakes becomes very saline over time such that its salinity is equivalent to, or sometimes exceeds that, of ocean water. Perhaps the most prominent example is the aptly-named Great Salt Lake in northern Utah.

Figure 23.4.3a (left) A salt-crusted playa at Death Valley National Park, California. Figure 23.4.3b (right) A temporary salt lake that has formed at Death Valley National Park.

In wrapping up the power of water in deserts, we’ve already discussed the deceptively powerful nature of relatively small amounts of water runoff on a desert surface (to form gullies). If enough water, from a storm, accumulates in tight, narrow slot canyons, the moving water pressure there becomes immense! The water is powerful enough to erode very large particle sizes (like boulders!). When the sediment-filled water escapes from a confined canyon and spills out into a wide open area, the large sediments are deposited in a sprawled out manner that resembles a fan. Hence, this depositional feature is called an alluvial fan. They are similar in nature to deltas (discussed in chapter 13). Both form when a channel (either a river or a canyon channel) terminates into a more open area. However, the end-stage portion of a river generally only erodes mud and silt. However, both features can radically be re-sculpted from a power storm or flood event.

Media Attributions

• Figure 23.4.1-3: Wikimedia Commons