desert sedimentary deposits

desert sedimentary deposits Today's deserts occur mainly either side of the Equator, between latitudes 10° and 30° in belts of high aridity associated with area of persistent high pressure. They are in addition found in the arid interiors of some continents, on the leeward sides of mountain ranges in rain shadows, and along coastal areas where the presence of cold oceanic water prevents transference of moisture to a neighbouring land-mass.

Most desert surfaces are erosional: they are formed of bare rock (hamada) or deflated stony plain (serirs or reg). However, large volumes of sediments accumulate in alluvial fans, aeolian sand seas (ergs), and in playa lakes or sabkhas.

Long rainless periods accompanied by diurnal temperature changes of large magnitude, together with crystallization of salts drawn to the surface by capillary action, combine to cover barren mountainous areas with a cloak of broken rock and mineral debris. Periods of violent rainfall, although rare, produce short intervals of intense run-off. Sediment-laden streams carve deep, steep-sided valleys which, where they debouch into adjacent lowlands, produce alluvial fans. These have the shape of a segment of a cone, with a profile that is convex upwards as seen across the fan and concave upwards along the fan profile. Surface slopes vary from 5° to 25°, but are usually less than 10°, and everywhere decrease away from mountain fronts. Adjacent fans may merge to form a broad sloping surface, known as a bajada, at the foot of a mountain range. They are crossed by a diverging pattern of shallow channels; streams flowing through the channels deposit cross-bedded imbricate gravels, sandy gravels, and sands which are lenticular, and commonly form down-fan longitudinal bodies. During extreme floods streams escape from the channels and deposit sheet-flow deposits, which consist of low-amplitude bars of clast-supported gravels and planar and cross-stratified sands in shallow channels over the whole fan surface. Where the source produces little fine material, open-work gravels—sieve deposits—are formed, which are often infilled by a later infiltration of finer sediment. The streams are sometimes so overloaded with sediment that they degenerate into viscous sediment gravity flows—mudflows—which may be confined to channels but, particularly in the upper fan spread over the fan surface. They produce matrix-supported conglomerates (paraconglomerates) that commonly show inversed grading with large boulders at the top of the deposit.

During intervening dry periods, winds scour the fan surfaces and sandblast exposed gravel clasts to produce smooth faceted ventifacts. Water drawn to the surface by capillary action precipitates iron and manganese crusts (‘desert varnish’) on the surface of the pebbles. Capillary movement of waters leads to precipitation of crusts of calcium carbonate (calcrete) or gypsum, (gypcrete). Such accumulations which develop in surface sediments mark hiatuses of deposition.

Apart from scattered plant roots, skeletal remains of terrestrial organisms, and some burrows, fossil remains are rare in desert deposits. Repeated flooding produces a thick wedge of coarse-grained deposits termed fanglomerates, which extend from the mountains into the adjacent lowlands. The sediments become finer and show increased roundness from the top to the bottom of the fan. Tectonic changes in the source area produce cyclic sequences (10–100 m thick) that coarsen upwards or sequences that become finer upwards where faulting has caused retreat of the mountain margin.

Although they cover only approximately 20 per cent of the world's deserts and are rare in some, aeolian sands are perhaps regarded as the most typical desert sediments. The sand seas or ergs cover thousands of square kilometres, and in places the sand cover may be hundreds of metres thick.

Wind winnows fine sediment from rocky, stony areas and the surfaces of alluvial fans to leave a gravel lag. The material of silt-sand size is transported in suspension to settle in playa lakes or sabkhas, or form loessic deposits on desert fringes.

Sand is transported by rolling, creep, saltation, and in suspension to form fine to medium, well-sorted well-rounded sands fashioned into a variety of bedforms. These grade from ripples (with heights ranging from centimetres to metres and wavelengths up to metres), dunes (with heights up to metres, wavelengths of hundreds of metres) and huge bedforms: draas (with heights up to hundreds of metres and wavelengths measured in kilometres) with superimposed dunes. Various dune forms develop, according to the wind regime and availability of sand.

Barchan, barchanoid dunes, and transverse dunes develop under winds with a low directional variability and show considerable lateral movement. Linear or seif dunes occur in areas of more variable winds and show less lateral movement; star-shaped dunes develop in areas of very variable winds and show little lateral movement and considerable vertical accretion.

Migration of dunes and deposition on their lee flanks produces cross-stratified sands up to 10 m or more in scale with moderately high angles of dip, and some superimposed ripple cross-stratification. Cross-stratification in barchan, barchanoid, and transverse dunes has a variable down-wind unimodal pattern. In linear dunes laminae dip away from the main axes to give a bimodal pattern, and in star dunes there is a polymodal pattern. The various patterns of cross-stratification have been used to differentiate various forms of dunes in ancient deposits.

As various bedforms migrate over one another, they produce a variety of erosional surfaces between sets or groups of cross strata; these are termed ‘bounding surfaces’. First-order bounding surfaces are formed where draas migrate over older aeolian sands; second-order bounding surfaces form where dunes move over the flanks of draas or one another; and third-order bounding surface are surfaces that cut across sets of cross-strata and are formed by erosion or changes in local wind direction.

In some deserts, salty flats (sabkhas) or ephemeral lakes or playas with marginal sabkhas occur. Wind-blown dust settles in these areas. They are sometimes fed directly by surface flow from adjacent fans or mainly by surface flow.

Evaporitic minerals (carbonates, gypsum, halite, borates, etc.) are precipitated from evaporating waters. The sediments show polygonal desiccation cracks and tepee structures—polygonal patterns of ridges formed by the expansive force of crystallization of the various salts.

As Fig. 1 shows, the general relationships between various facies—alluvial fan, aeolian sands and playa deposits—depend on the tectonic setting. In many fault-bounded desert basins (e.g. the western USA), fans pass directly into playa lake sediments with little dune sand. In other deserts (e.g. Arabia) alluvial fans are bounded by wide dune fields with few playas.

Desert sediments are well known in the geological column. Although they are commonly red or buff in colour with low contents of organic matter, their coloration is mainly of diagenetic origin and is not exclusive to these deposits. It cannot therefore alone be used as an environmental indicator of arid conditions. The Permian deposits of the North Sea show the development of alluvial fans, aeolian sands, and playa lakes in a most striking way (Fig. 1c).

G. Evans

Bibliography

Glennie, K. W. (1970) Desert sedimentary environments. Elsevier, Amsterdam.