Deserts
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[edit] Introduction
In this article we shall discuss the forces that shape deserts, and discuss how geologists can use this knowledge to identify rocks that are the lithified remains of former deserts. We shall use some terms that have been introduced in previous articles on Sedimentary Rocks and Mechanical Weathering and Erosion; the reader who has not already read these articles may find it useful to go back and do so.
[edit] Causes of deserts
The immediate cause of a desert is, by definition, lack of rainfall. This itself can have a number of causes, which are not mutually exclusive: an area can be a desert for more than one reason.
- High-pressure deserts. In zones of high atmospheric pressure, the ability of air to contain moisture is increased, resulting in little rainfall. Examples include the Sahara, Arabian, Thar, and Kalahari deserts, and the desert regions within the Arctic and Antarctic circles.
- Mid-continent deserts. Areas in the middle of a continent can receive little rainfall simply because rain, originating from evaporation of seawater, will tend to fall before it can reach the middle of a large continent. Modern examples are the Turkmenistan, Gobi, and Great Australian deserts (the Great Australian Desert is also in a region of high pressure).
- Rain-shadow deserts. Rain will tend to break over mountains, so the presence of mountains can prevent rain from the sea from coming inland. Examples of rain-shadow deserts include the Mojave desert in the rain-shadow of the Sierra Nevada, the Patagonian desert in the rain-shadow of the Andes, and the Iranian desert in the rain-shadow of the Zagros mountains.
- Upwelling deserts. Finally, a desert may be by the coast and not in the rain-shadow of any mountains, but be adjacent to where a cold current of water rises to the ocean surface, reducing evaporation. Examples Include the Atacama desert, the Western Sahara, and the Namib desert; these are all also in high-pressure zones.
[edit] Deserts and water
Although rainfall is rare in deserts, its effects are important in understanding the geology and ecology of deserts.
[edit] Alluvial fans and bajadas
Two or more adjacent alluvial fans can merge to form what is known as a bajada: one is shown in the photograph to the left, taken in Death Valley. The braided fan-like patterns made by the streams that deposited it are clearly visible.
In the foreground of the photograph, note the white deposits of salt. Although most of the insoluble sediment is deposited at the foot of the mountain, the water will of course retain its dissolved mineral content until it evaporates, leaving behind the minerals as an evaporite deposit.
[edit] Playa lakes
If the water contains any dissolved minerals these will be deposited on top of the clay layer as the water evaporates, leaving an evaporite bed of such minerals as halite, calcite, gypsum, borax, or trona, depending on the rocks that were their source. Repetition of this process builds up alternating layers of detrital and evaporite sediments.
[edit] Erosion by water
Despite the fact that rain is rare in deserts, when it does arrive, it is often intense, causing flash floods. This can make it a powerful erosional force.
[edit] Oases
Oases are small lakes in the desert. These have a number of causes: they can be fed by springs; they can be deflation lakes, where erosion has caused a hollow the bottom of which lies below the water table; or they can be intermittent lakes filled by occasional rainfall and runoff.
[edit] Deserts and wind
Wind has a number of effects. First, it can remove the sand and other light particles from the surface. leaving bare rock or the stony mosaic surface known as a desert pavement (see the photograph to the left.)
Second, it can erode rocks by abrading them with the particles it carries, as discussed and illustrated in our main article on Mechanical Weathering and Erosion.
Thirdly, it can pile sand up into dunes, giving us our stereotypical image of a desert. A sandy desert is known as an erg.
The result of the transport of grains from stoss to lee is that the dune will move downwind. However, it will not always erode completely on the stoss side, instead leaving behind it a set of cross-beds composed of the bottom of the grainflow laminae, as we have shown in our diagram. Then the next sand dune to pass that way can deposit another discontinuous set of cross-beds on top of that set (and on top of any sand that may have been deposited by the wind between the passing of the two dunes). By this process, set after set of cross-beds build up.
The cross-bedding is informative in a number of ways. First, it shows us that the sand was deposited in dunes: we shall discuss in the next section how we can determine that these dunes were caused by currents of wind and not water.
Secondly, you will note from our diagram that the gradient of a grainflow lamina tends to become shallower at the bottom: this feature can be seen in the cross-beds in the photograph. Now, tectonic events are quite capable of turning rocks sideways or upside-down. The curves of the cross-beds are one of a number of features, known as way-up marks, that allow geologists to determine which way was originally up: the curves are concave in the up direction, convex in the down direction. In the case shown in the photograph, we can see that the sandstone is still the right way up.
Thirdly, the cross-beds show us the prevalent direction of the winds: in this photograph we can see that they must have been blowing from the left to the right of the picture.
[edit] Lithified deserts: how do we know?
There are a number of features that we can use to identify sedimentary rocks that are the lithified remains of ancient deserts. In this section we shall review some of them.
Desert sand is quartz or arkose, and the resulting sandstone will be quartz or arkose sandstone. This is not a definitive criterion for recognizing aeolian sandstone, since quartz sandstone can be formed under other conditions. We may, however, safely say that graywacke is not aeolian sandstone.
Desert sands tend to be well-rounded, as a result of long abrasion, and well-sorted by size, and this is what we find in the grains of aeolian sandstone.
Much, though not all, desert sand is colored, the grains being stained on the outside with iron oxides such as hematite, goethite and limonite, giving it a range of colors through red and orange to yellow. However, some desert sands are not stained in this way, so we cannot definitively say that sandstone lacking this feature is not desert sand.
Cross-bedded features reveal transport by a current of wind or water. We can distinguish between the two cases by various features: for example, wind-formed dunes result in much bigger sets of cross-beds; also, aeolian sandstone uniquely exhibits pinstripe laminae, very narrow stripes only a few grains thick consisting of finer grains than are found in grainfall laminae.
Playa deposits are extremely distinctive, and leave geologists no doubt that what they are looking at was once desert: they can only be formed by repeated episodes of deposition in and evaporation of a playa lake.
Finally, we can look at the fossils, which, in a desert should be of land plants and animals, and of freshwater organisms in those rare places where the geology indicates a former oasis. No marine fossils should be found.
[edit] Case study: the Navajo sandstone
- The sand, which is cemented together by calcite, silica, and hematite, is very pure quartz sandstone without clays or feldspars.
- The sandgrains are well-sorted, consisting mainly of medium-sized grains; and well-rounded. The photograph to the right shows a microscopic view of grains from a graniflow lamina in the Navajo sandstone that have been crumbled gently by hand from the surrounding matrix.
- In most of the Navajo sandstone, the grains are richly stained with iron oxides; this again can be seen in the photograph to the right.
- The sandstone shows large cross-bedded features, of a type only observed to form in desert winds. The photograph showing cross-bedded sandstone in the section on deserts and winds, above, is from the Navajo formation.
- The sandstone exhibits pinstripe laminae, uniquely characteristic of aeolian sandstone.
- The sandstone contains layers of evaporites characteristic of playa lakes.
- The sandstone contains no marine fossils, but instead contains fossils of land reptiles including Protosuchus, Ammosaurus, and Segisaurus. It contains fossilized footprints of land animals. It contains fossilized land plants including conifers, horsetails and ferns; root casts and stumps of trees can be found.
- The land plants and animals and their footprints are found in association with rare thin interdune deposits consisting of lenses of limestone containing freshwater shells, indicating the presence of temporary freshwater oases (temporary, that is, in terms of a geological timescale).[1]
In short, the Navajo sandstone was once a large sandy desert.
