Desert sediments: ancient and modern

Tropical deserts have existed sporodically on our planet from the Precambrian to the Present, and seem not to have been a permanent feature of its surface (Glennie 1987). Depending on their definition, tropical deserts and semi-deserts currently occupy between approximately one fifth and one third of the Earth's land surface, of which only about 4 ~ is covered by that popular concept of what a desert consists of--sand dunes. The remaining area comprises barren rock (both hill and plateau) with a variable cover of sediments transported by ephemeral streams (wadis, arroyos) to form the deposits at the terminal points of these streams. Varying with the ratio of water supply to the annual potential rate of evaporation, those terminal areas may be occupied by desert lakes that are generally of a temporary nature (permanent only if the water is provided from beyond the margins of the desert or is fed by groundwater), and become more saline as they become desiccated. The end product of such a situation is a salina or sabkha (area of sand, silt or clay, commonly encrusted with halite). There is no universally accepted definition of a desert. In its simplest form it can be defined as a barren tract of land over which rainfall is too limited or spasmodic to support vegetation adequately. Very few desert areas are completely devoid of vegetation, and many areas that fall within a desert in terms of average annual rainfall may have an even though sparse cover of plants that have adapted to the relatively arid environment in which they live. In this context, some writers define deserts as areas that have an average upper limit of 250 mm of annual rainfall, even though it may all fall in one storm and rainfall may not recur for several years. Perhaps more important than the rainfall itself is the ratio between it and the potential rate of evaporat ion-the aridity or desiccation factor. This is about 1 : 10 in some parts of the Australian Desert and up to 1:500 in areas of the Sahara (Cooke & Warren 1973). Tropical deserts exist either because the winds that cross their surfaces have a low relative humidity or there are no hills to push the winds up to colder cloud-forming altitudes; the Southwest Monsoon gives torrential rain over the Western Ghats of peninsular India but little over the arid low-lying Rajasthan Desert further to the N. Drawn from the high-pressure area of the Mediterranean Sea, the North African trade winds blow south and southwestward under clear skies, warming as they cross the Sahara, and so becoming capable of absorbing more moisture without condensation. By contrast, the southern hemisphere coastal deserts of the Namib and Atacama are often shrouded in mist but rarely experience rain. Cold deserts also occur in areas of low precipitation but lack a protective cover of vegetation because the moisture necessary for growth is frozen for a large part of the year. Allied to the low temperature, peri-glacial dune fields exist because of adequate supplies of fluvioglacial sediments that are unprotected by vegetation and have a grain size that is suitable for deflation once the summer melt-floods have ceased. Dunes are formed because strong dry winds can transport sand-size particles as a saltation load within a few metres of the land surface (sand storm). Because winds can drive sand over a hard immobile surface faster than over one of loose sand (Bagnold 1941), sand tends to accumulate in areas that are already sand covered. These sand patches grow in size whenever the velocity of the wind lessens and is no longer capable of keeping the grains in motion; dunes eventually result. Silt and clay-size particles, however, can be carried in suspension (dust storm) and by this means may be transported right out of the desert to be deposited elsewhere, as a marine horizon in the ocean or on land as loess. A cover of vegetation on the land surface tends to inhibit the aeolian transport of sand. Some ancient dune sequences that were deposited before higher plants began to colonize extensive areas of land during the Devonian (eg Precambrian dunes; Ross 1983) may reflect the ease with which sand could be moved by the wind in the absence of vegetation rather than any long-term aridity. The Devonian Old Red Sandstone sequences of Britain, for instance, are dominated by fluvial and lacustrine sediment that must indicate a fairly high annual rainfall, and yet dune sands have accumulated locally (Mykura 1983; Carruthers, this volume). To confirm a true desert origin for ancient sequences, additional evidence of aridity is needed. In hot deserts this is most easily recog-