Thursday, 7 January 2016

Deposition in Deserts

Deposition in Deserts

We've seen that erosion relentlessly eats away at bedrock and sediment in deserts. Where does the debris go? Below, we examine the various desert settings in which sediment accumulates.

Talus Aprons 

Production and transportation of debris and sediment in deserts.
Over time, joint-bounded blocks of rock break off ledges and cliffs on the sides of hills. Under the influence of gravity, the resulting debris tumbles downslope and accumulates as talus, a pile of debris at the base of a hill. Talus can survive for a long time in desert climates, so we typically see aprons of talus  fringing the bases of cliffs in deserts (figure above a).

Alluvial Fans 

Flash floods can carry sediment downstream in a steep-walled canyon. In some cases, the water contains so much sediment that it is best called a debris flow. When the turbulent water flows out into a plain at the mouth of a canyon, it spreads out over a broader surface and slows. As a consequence, sediment in the water settles out. The channel that emerges from the mountains tends to subdivide into a number of subchannels or distributaries that diverge outward. The network of distributaries spreads the sediment, or alluvium, out into a broad alluvial fan, a wedge- or apron-shaped pile of sediment (figure above b). Alluvial fans emerging from adjacent valleys may merge and overlap along the front of a mountain range, producing an elongate wedge of sediment called a bajada. Over long periods, the sediment of bajadas can fill in adjacent valleys to depths of several kilometres.

Playas and Salt Lakes 

Playas form where a shallow, salty lake dries up.
Water from a flash flood occasionally reaches the centre of an alluvium-filled basin. But if the supply of water is relatively small, the water quickly sinks into the permeable alluvium without accumulating as a standing body. During a particularly large storm or an unusually wet spring, however, a temporary lake may develop over the low part of a basin. During drier times, such desert lakes evaporate entirely, leaving behind a dry, flat, exposed lake bed known as a playa (figure above a, b). Over time, a smooth crust of clay and various salts (halite, gypsum, borax, and other minerals) accumulates on the surface of playas.
Where sufficient water flows into a desert basin, it creates a permanent lake. If the basin is an interior basin, with no outlet to allow water to flow out, the lake becomes very salty, because although its water escapes by evaporation in the desert sun, its salt cannot. The Great Salt Lake, in Utah, exemplifies this process. Even though the streams feeding the lake are fresh enough to drink, their water contains trace amounts of dissolved salt ions. Because the lake has no outlet, these ions have become concentrated in the lake over time, making it even saltier than the ocean.

Deposition from the Wind 

As mentioned earlier, wind carries two kinds of sediment loads a suspended load of dust-sized particles and a surface load of sand. Much of the dust is carried out of the desert and accumulates elsewhere. Sand, however, cannot travel far, and accumulates within the desert in piles called dunes, ranging in size from less than a meter to over 300 m high. In favourable locations, dunes accumulate to form vast “sand seas.”
Figures credited to Stephen Marshak.