Hot Desert Environments

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Processes and Products

Wadi conglomerates

Desert stream channels are known as wadis. Their source is usually high in mountains, so they have steep gradients. Together with the intensity of the rainfall, this gives the streams very high energy and so they can transport coarse grained fragments. Energy is quickly lost when the rain stops and because water sinks into the porous rocks of the channel. Deposition is rapid, leaving poorly sorted conglomerates. Grains may be sub-rounded if they have been transported on several occasions. Grains that have only been briefly transported, perhaps during one flash flood, may be angular. Desert conditions often lead to grains having a red coating of oxidised iron minerals.

Desert Sandstones

Sand grains are transported by high energy winds and affected by attrition. This makes them:

very well sorted

very well rounded

high sphericity

frosted due to the collisions between grains

They are composed entirely of quartz grains because other minerals present in the source rocks have been removed mechanical and chemical weathering. Quartz remains because it is unreactive and hard. The frosted surfaces of the grains are due to attrition during transport. Because of their shape and size they are sometimes described as 'millet-seed sand'.

Desert sandstones are formed from these sands. The grains are coated with oxidised iron minerals so that they have a red colour and desert sandstones often have silica cement. They show large scale cross bedding on a scale of metres rather than centimeters because they are often deposited in large dunes. The dunes may be crescent shaped barchan dunes or straight seif dunes. The dunes move in the direction of the prevailing wind and sand is blown up the windward side and slips down the leeward side. It is this face that can be preserved to make cross bedding. Some dunes are 200m high.

Playa Lakes Evaporites

Due to the hot climate, infrequent rainfall, high rates of evaporation and infiltration, desert streams tend not to flow all the way to the sea. Hot deserts are often regions of inland drainage where streams flow into temporary playa lakes. Stream water flowing into the lakes contains lots of ions of calcium, sodium and potassium in solution. They are products are the weathering rocks upstream. When the water evaporates in the hot sun, all of the dissolved ions become more concentrated. The least soluble are the first to precipitate out to form layers of evaporite minerals. The most soluble will be in the centre of the playa lake.

Precipitated first:

calcite - white, hardness 3, rhombic crystals and good cleavage, least soluble

gypsum - white, hardness 2, crystals or may form nodular layers

halite - cubic 'hopper' crystals, cubic cleavage, salty taste

k minerals - most soluble

Fine grained sediment is also deposited from suspension to form mudstones in playa lake environments. These may contain desiccation cracks, ripple marks and salt pseudomorphs, as well as lenses of evaporites, usually gypsum.

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