Page 139 - Sedimentology and Stratigraphy

126 Aeolian Environments

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Fig. 8.16 During glacial periods the regions of polar high pressure are larger, creating stronger pressure gradients and hence
stronger winds. In the absence of large high pressure areas at the poles in interglacial periods the pressure gradients are
weaker and winds are consequently less strong.
they are also strongly oxidising environments, so carried in the ice were released. In the cold periglacial
plants and animals are likely to completely decompose environment in front of the receding ice colonisation
and leave no fossil material. Only the most resistant by plants and stabilisation of the soil would have been
animal remains, such as the bones of large animals slow, so the glacial debris was exposed on the out-
such as dinosaurs, have much potential to be pre- wash plains, where wind picked up and transported
served in aeolian environments. Trace fossils (11.7) the silt-sized dust. This dust was probably transported
are also rare because few animals live on active sand over large parts of the globe but accumulated as loess
dunes, but there is the possibility of walking traces deposits in some places. Similar processes probably
being preserved in fine sediment in wet interdune occurred during other glacial episodes in Earth his-
areas. Strata deposited in desert environments are tory, but pre-Quaternary loess deposits have not been
therefore likely to be barren of any fossils. recognised. The preservation potential of loess is likely
to be quite low because it is soft, loose material that is
easily reworked and mixed with other sediment.
8.6 AEOLIAN DEPOSITS OUTSIDE Volcanism is an important source of dust in the
DESERTS atmosphere. Explosive eruptions can send plumes of
volcanic ash high up into the atmosphere where it is
8.6.1 Aeolian dust deposits distributed by wind. Coarser ash tends to be deposited
close to the volcano (although in very large eruptions
There are deposits of Quaternary age in eastern Eur- this can be hundreds of kilometres away – 17.6.2),
ope, North America and China that are interpreted as while the silt-sized ash particles can be transported
accumulations of wind-blown dust (Pye 1987). These around the world. Large amounts of atmospheric dust
deposits, known as loess, locally occur in beds several from eruptions can darken the sky, and it will gradu-
metres thick made up predominantly of well-sorted ally fall as fine sediment. A further source of atmo-
silt-sized material, with little clay or sand-sized mate- spheric dust is from fires that propel soot (fine carbon)
rial present. The origin of loess is related to episodes of up into the air, where it can be redistributed by the
retreat of ice sheets, as large amounts of loose detritus wind. Despite the fine grain size, soot, volcanic and