10.2: Deserts
- Page ID
- 26672
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Numerous landscapes and landscapes and landforms exist in deserts and the geomorphic study of these features is beyond the scope of this book. Instead, we will focus on the features of deserts that are most commonly preserved in the geologic record.
Although desert conditions can occur at just about any geographic location, areas that are ~30° from the equator tend to be at least semiarid because this is where we have convection cells bringing dry air down from altitude. Deserts are also particularly likely to form in "rainshadow" areas in the downwind side of mountain ranges.
In the geologic record, desert deposits can cover thousands of square kilometers and be many hundreds of meters thick. By definition, deserts are dry areas that receive less than 10 inches of rain per year; limited water (and thus limited plants) means that sediment transport by wind tends to be the dominant process. Silt and clay sized particles can be suspended within the air and larger sand-sized particles can be temporarily suspended as well as moved by traction and saltation.
Windblown sand can be organized into large sand dunes with variable morphologies; they can be up to tens of meters tall and are often preserved as wedge-shaped sets of cross beds. Low-amplitude wind-ripples migrate across dune faces. When sand accumulations become oversteepened or disturbed, grainflow can happen down dune surfaces which causes inverse grading and cross-laminae that thicken downdip.
Interdune areas can record a variety of processes and are more sedimentologically complex. Common features in interdune areas include:
- Deflation lags - accumulations of gravel that form when finer sediment is winnowed away leaving a gravel-armored surface behind.
- Ventifacts - faceted clasts that are created by wind erosion
- Ephemeral lakes - temporary bodies of water that can leave behind evaporites, mud, and evidence of dessication.
- Interdune vegetation - plants adapted to arid conditions can take root in the more stable substrate of interdune areas
- Fluvial systems - can be locally or seasonally present, especially where rivers are sourced from more distant or upland areas. Fluvial transport of sediment into deserts provides input for sediment that will eventually be reworded and transported by the wind.
Overall, desert deposits commonly consist of large, eolian cross-beds that can be much thicker than subaqueous cross-beds, low amplitude wind ripple cross laminae, deflation lags, evidence of ephemeral lakes and rivers, evaporite minerals in playas or paleosols.