Page 138 - Sedimentology and Stratigraphy

Desert Environments 125

Fig. 8.15 The global distribution of modern deserts: most lie within 408 of the Equator.

Some modern desert sands are strongly oxidized, with are deposited in other environments which are also
haematite coatings on the grains giving them a vivid shades of red: for example, mud deposited in deep
orange-red colour. Green, grey and black sediments oceans (16.5) may contain aeolian dust that includes
are unlikely to be found in desert environments haematite, giving the sediment a reddish colour.
because these colours are due to reduced iron oxide
(FeO, which is green) and the presence of dark organic
material preserved in the sediment: preservation of 8.5.4 Life in deserts and fossils
these materials is unlikely in an environment that is in aeolian deposits
mainly dry and exposed to the air, and therefore
strongly oxidising. The absence of regular supplies of water in deserts
Sedimentary successions composed of strongly red- makes them harsh environments for most plants and
dened sandstone and mudstone are sometimes referred animals. A few specialised plants are able to survive
to as red beds (Turner 1980). It is tempting to use the long periods of drought and these form the bottom of
presence of haematite as evidence that the sediments the food chain for insects, reptiles, birds and mam-
were deposited in an oxidising continental environ- mals, but none occur in large quantities. The inter-
ment. However, some caution is needed when using dune areas are the most favourable places to support
the colour of the beds as an indicator of depositional life because these can be places where water tempo-
environment. First, not all desert deposits are red in rarily ponds and are the closest points to any ground-
the first place, and second, the colour may be the water if the water table is relatively close to the
result of oxidation after deposition (a diagenetic pro- surface. In terms of fossil preservation, the paucity of
cess – 18.2.4). There are also cases of sediments that organisms in deserts is compounded by the fact that