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.

Figure \(\PageIndex{1}\): Photograph of Death Valley National Park showing many of the striking landforms present in desert areas (Brocken Inaglory via Wikimedia Commons; CC BY-SA 4.0).  The geomorphology of these areas is beyond the scope of this chapter; instead we will focus on the features of desert landscapes that are most commonly preserved in the geologic record.

 

Figure \(\PageIndex{2}\): Atmospheric circulation patterns showing how circulation cells can influence precipitation patterns at different latitudes (modified from Kaidor via Wikimedia Commons; CC BY-SA 3.0). 

 

Figure \(\PageIndex{3}\): Illustration of the rainshadow effect on the downwind side of mountain ranges.  

 

Figure \(\PageIndex{4}\): Map showing the distribution of vegetation types; deserts are represented as browns and tans (modified from Ville Koistinen (user Vzb83) via Wikimedia Commons; CC BY-SA 3.0).  Notice that deserts are particularly abundant and on 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 mi² extent of the Navajo Sand Sea. And even this vast ancient desert is dwarfed by the modern Sahara Desert which covers >3.6 million mi² in northern Africa.  

Figure \(\PageIndex{5}\): Exposures of hundreds of meters of the Navajo Sandstone (Jurassic) in the walls of Pine Creek and Zion Canyons, Zion National Park (Steve Boland via Flickr; CC BY-NC-ND 2.0).

 

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.

 

Figure \(\PageIndex{6}\): Photograph of a sandstorm about to overtake a military base in Iraq (Tobin via Flickr; CC BY-NC-SA 2.0).  These impressive storms form when winds grow strong enough to suspend and/or saltate silt- and clay-sized particles exposed at the surface.  

 

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.

 

 

Figure \(\PageIndex{7}\): Modern sedimentary structures in deserts (left) and corresponding sedimentary structure preserved in the rock record (right).  A) Large sand dunes in White Sands National Park ornamented with low-amplitude wind ripples oblique to the crest of the dune (Michael C. Rygel via Wikimedia Commons; CC BY-SA 3.0).  Note that the dune is migrating into an interdune area with relatively abundant vegetation.  B) Large eolian cross-beds in the Jurassic Najavo Sandstone in Zion National Park (Michael C. Rygel via Wikimedia Commons; CC BY-SA 3.0).  Circled area shows a person for scale.  Because the ultimate limiting factor on the height of dunes is the depth of the fluid, windblown dunes in deserts can grow to impressive heights.  C) Grainflow in cohesionless sand on the slipface of a dune; footprint for scale  (Michael C. Rygel via Wikimedia Commons; CC BY-SA).  These features would be preserved as wedge shaped layers of sediment that thickened in a downslope direction; they might contain inverse grading.  D) Inverse grading in eolian sandstone of the Jurassic Carmel Formation (Michael C. Rygel via Wikimedia Commons; CC BY-SA). 

 

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.

 

Figure \(\PageIndex{8}\): Modern sedimentological feature of interdune areas.  A) Deflation lag (desert pavement) in Death Valley National Park (Michael C. Rygel via Wikimedia Commons; CC BY-SA 3.0). B) Ventifacts with flat, faceted faces formed by wind erosion in a desert in Libya (calind via Wikimedia Commons; CC BY-SA 3.0). C) Oblique Google Earth image showing an interdune area in the Namib Sand Sea with a seasonal fluvial system and interdune vegetation (courtesy Google Earth, exported images from Google Earth can be embedded on websites for educational and non-commercial use). D) Photograph of interdune area with vegetation and desiccation cracks in fine grained sediment deposited in an emphemeral lake (Anthony Brown via Pexels; Pexels license).

 

Figure \(\PageIndex{9}\): Diagram showing the formation of a deflation lag (desert pavement) by the removal of fines via wind and concentration of remaining coarse fraction (Page Quinton via Wikimedia Commons; CC BY-SA 4.0).

 

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.

Figure \(\PageIndex{10}\): Overview of the sedimentology of desert deposits with particular emphasis on features that are likely to be preserved in the geologic record (Page Quinton via Wikimedia Commons; CC BY-SA 4.0).

 

Readings and Resources

• Laity, J.J., 2008, Deserts and Desert Environments, Wiley-Blackwell, 368 p., ISBN: 978-1-577-18033-3.

• 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.