10.2: Deserts
Definition and Distribution
By definition, deserts are dry areas that receive less than 10 inches of rainfall per year. Although these 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.
Processes and Deposits
In the geologic record, desert deposits can cover thousands of square kilometers and be many hundreds of meters thick. The Jurassic Navajo Sandstone and correlative units reach thicknesses of up to 700 m and widespread occurrences across Wyoming, Utah, Colorado, and Arizona represents under half of the original >340,000 mi 2 extent of the Navajo Sand Sea. And even this vast ancient desert is dwarfed by the modern Sahara Desert which covers >3.6 million mi 2 in northern Africa.
In terms of the origin of sediment within deserts, it may be derived from weathering of older (underlying) rock and sediment, fluvial transport or mass wasting from adjacent uplands, or moved by wind from other areas within the desert. The lack of water in these arid areas limits plant growth, which means that under normal conditions sediment transport is dominated by wind. 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 very large, wedge-shaped sets of cross beds. Low-amplitude wind-ripples migrate across dune faces. When sand accumulations become oversteepened or disturbed, grain flow can happen down dune surfaces which causes inverse grading and cross-laminae that thicken downdip.
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Figure \(\PageIndex{7}\): Collection of videos showing common sediment transport processes in deserts. Clockwise from the top left they include suspension of silt and sand in a storm, saltation of sand grains, grainflow down the slipface of a dune, and movement of sediment into and across a desert by flash floods.
Interdune areas can record a variety of processes and are more sedimentologically complex. Common features in interdune areas include:
- Deflation lags (or desert pavements ) - 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
- Playa lakes - temporary bodies of water that can leave behind evaporites, mud, and evidence of desiccation.
- 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.
Overview
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.
Readings and Resources
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Laity, J.J., 2008, Deserts and Desert Environments, Wiley-Blackwell, 368 p., ISBN: 978-1-577-18033-3 .
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Brookfield, M. E., & Silvestro, S. (2010). Eolian systems. In N. P. James & R. W. Dalrymple (Eds.), Facies Models (4th ed., pp. 7–7). Geological Association of Canada.