playas The flattest, smoothest geomorphological surfaces on Earth are playas. Commonly composed of compact silt and clay, sometimes with a surface salt efflorescence, they are often ideal for high-speed car racing as in the case of the Bonneville salt flats in Utah. However, they can also be crossed by giant desiccation cracks that have been known to catch unwary drivers by surprise. But how do playas form, what lies beneath them, and what processes are active on their surfaces?

Playas (salinas in South America; sabkhas or sebkhas in Africa) are found in closed interior basins, or bolsons, in arid regions (Fig. 1). In such areas (the western United States is a good example) differential movements of the Earth's crust or other processes have resulted in depressions surrounded by mountains. Under more humid climates in the past, many of these depressions contained perennial lakes. At present, however, water reaching the basin floor evaporates within days or weeks.

Surrounding a typical playa, and sloping towards it, are deposits of gravel called alluvial fans (Fig. 1). The apices of these fans are at the mouths of valleys carved into the mountains. The boundary between the alluvial fans and the playa is normally a few tens of metres wide. Over this distance the slope decreases abruptly.

Role of water under present climatic conditions

Storms in arid regions, although infrequent, are often intense, and the sparse vegetation does little to inhibit run-off or erosion. Consequently, floods coursing out of the mountains typically carry a heavy load of sediment. When these floods reach the toe of a fan, the break in slope instigates deposition of the coarser material—that carried as bed or traction load. Waves of deposition propagate up the fan from this break in slope. (Contrary to some early opinions, there is rarely a break in slope in the stream profile at the fan apex.)

Water continuing out over the playa surface carries with it a quantity of fine sand, silt, and clay in suspension. This sediment is deposited as the water evaporates. Rates of sedimentation may range from a few centimetres to about a metre per thousand years. Because the water fills the lowest parts of the bolson, sedimentation is concentrated there and serves, over the course of millennia, to level the playa surface. This is unlike the situation in perennial lakes where sedimentation also occurs on sloping margins, thus perpetuating variations in depth.

Playa area

Playas typically occupy 2 to 6 per cent of the depositional area in a bolson; alluvial fans make up the remainder. This reflects the size distribution of material supplied by the mountains. Were there no silt and clay in the sediment load, or only enough to fill interstices in the gravel on the fans, there would be no playa. Conversely, geological terrains yielding higher percentages of silt and clay in comparison with coarser material have playas that occupy a larger fraction of the depositional area.

Surface processes

A playa surface may be either ‘wet’ or ‘dry’. Both types of surface may coexist on the same playa. A wet playa is one beneath which the water table is so shallow that water is drawn to the surface by capillary action. As the water evaporates, salts (particularly halite) are precipitated. Initially, this results in a surface coating of delicate white crystals or, occasionally, a puffy dry porous surface. Over time-spans of decades, a layer of silty salt that is centimetres or even decimetres in thickness may develop. Because crystallization of salt in such a layer involves lateral expansion, pressure ridges form. These are normally arranged in a polygonal pattern. The polygons range from one to a few metres across. Flooding of a playa by a rainstorm dissolves the salt and ‘resets the clock’ on this process.

Dry playas are those beneath which the water table is at some depth. After flooding and desiccation, newly deposited mud on such a playa dries and contracts, forming mud cracks. As the mud contracts, it commonly detaches along thin layers of fine sand that settled out of the water column just before the muds. The area and thickness of the individual mud blocks isolated by such cracks is controlled by the depth to these weak sand partings. Decimetre-scale blocks, for example, may be associated with sand lamellae a few centimetres beneath the surface, while chips a centimetre across may be only a millimetre thick. Because contraction is greatest in clays that settle out last, chips less than a few millimetres thick commonly curl.

As mentioned, giant desiccation cracks, up to a metre wide and metres deep, may also form. These appear to be associated with deep and declining water tables (lowered by pumping) and prolonged periods of drying.

Shallow channels may form on some playas, particularly if the playa surface is sloping. In Death Valley, California, for example, tectonic tilting is taking place faster than deposition can relevel the playa surface. Thus, water reaching the playa continues across it towards the lowest point, and flows in this direction persist long enough to erode and maintain channels that are metres in width and decimetres deep.

Stratigraphy

Under present climatic conditions, the sediment accumulating on playas is normally silty clay with little if any stratification. The lack of stratification is probably a consequence of the mixing of old and new sediment whenever the playa is flooded. On playas with channels, overbank flooding may leave thin layers of fine sand, silt, and clay.

Many bolsons, however, contained perennial lakes during the ice ages. Boreholes in these playas reveal a more complicated stratigraphy, consisting of tan, green, and black muds, fine sands, and evaporites (mostly halite, less commonly gypsum, calcite, or other minerals). Near the playa margin, gravels may also be encountered. Study of such stratigraphy can elucidate the history of the lake phases and can yield information on the climate at the time when the sediments were deposited. Black clays, for example, are evidence of anoxic conditions and diagnostic of a perennial lake. Thick evaporites indicate a period of desiccation, and detailed study of them can provide estimates of palaeotemperatures. Interbedded mud and halite layers are characteristic of periodic flooding in a salt-pan environment such as is found in Death Valley today. Where such sedimentary units can be correlated between two or more boreholes, it is sometimes possible to show that the floor of the former lake basin was not as flat as the present playa surface. Correlation of post-Pleistocene beds may also provide information on rates of tectonic deformation.

Roger LeB. Hooke

Bibliography

Hooke, R. LeB. and and Dorn, R. I. (1992) Segmentation of alluvial fans in Death Valley, California: New insights from surface exposure dating and laboratory modelling. Earth Surface Processes and Landforms 17, 557–74.
Jansson, P.,, Jacobson, D.,, and and Hooke, R. LeB. (1993) Fan and playa areas in southern California and adjacent parts of Nevada. Earth Surface Processes and Landforms, 18, 109–19.