Page 136 - Sedimentology and Stratigraphy

Desert Environments 123

Arid zone between sea level and sediment accumulation in
other environments is considered in Chapter 23.
Scale Lithology MUD SAND GRAVEL Structures etc Notes
clay silt vf m vc gran pebb cobb boul 8.5.2 Global climate variations
f
c
The formation of ergs requires an appropriate config-
uration of topography and wind patterns within a
suitable climate belt. Modern sandy deserts are in the
Aeolian sand
dunes. Well-sorted warm subtropical regions, which have predominantly
cross-bedded
sands dry, offshore wind patterns: most, in fact, lie to the
western sides of continents in belts of westerly winds
that have lost all of their moisture while crossing the
eastern side of the continent (Fig. 8.15). Although
similar conditions are likely to have existed at many
times and places through Earth history, the number
and extent of sandy desert areas are likely to have
varied as plate movements rearranged the continents.
Global climate is also known to have changed
through time. There have been periods of ‘green-
house’ conditions when the temperature worldwide
was warmer, and ‘icehouse’ periods when the whole
world was cooler. During ice ages large ice sheets
10s metres Alluvial fan formed on one or both of the polar regions. The
deposits.
Matrix-supported
conglomerates
deposited by debris increased areas of ice created larger areas of high
flows pressure, and there would have been steep pressure
gradients between the expanded polar regions and the
lower pressure equatorial belt. These conditions
resulted in belts of strong winds in the subtropical
regions, and hence increased potential for aeolian
transport and deposition (Fig. 8.16). The large ergs
of some modern deserts may be relics from the Pleis-
tocene when they were very active, but have since
become largely immobile. It is also notable that there
are extensive aeolian deposits in the Permian of
northern Europe, a time of Gondwana glaciation in
Ephemeral lake
deposits. Thinly the southern hemisphere.
bedded couplets of
mudstone and
evaporites
8.5.3 Colour in desert sediments
The sands in modern deserts such as the Sahara are
generally yellow. This colour is due to the presence of
Fig. 8.13 Graphic sedimentary log of the arid-zone envi- iron minerals, which occur as very fine coatings to
ronments shown in Fig. 8.12. the sand grains, particularly the iron hydroxide
goethite (3.5.1) (Fe(OH) x ), which is a dull yellow
the level of the interdunes will make more sediment mineral. Oxidation of goethite forms the common
available to be transported by the wind and this iron oxide mineral haematite, Fe 2 O 3 , and this very
material may be removed from the area of the erg, common mineral has a strong red colour when it is
that is, there will be net erosion. The relationship very finely disseminated as a coating on sand grains.