Page 160 - Sedimentology and Stratigraphy

Soils and Palaeosols 147

are closely associated with the vertical water move- . oxidising/reducing conditions (redox conditions),
ment as they involve the transfer of dissolved material particularly with respect to iron minerals;
from one layer to another, the formation of new . the development of layering (horizonation);
minerals and the breakdown of some original mineral . the redistribution of clays, iron and organic mate-
material. The activity of plants is evident in most soils rial into these different layers (illuviation);
by the presence of roots and the accumulation of . the amount of organic matter that is preserved.
decaying organic matter within the soil. The activity Twelve basic types of soil can be recognised using
of animals can have a considerable impact, as verte- the US Soil Survey taxonomy (Retallack 2001)
brates, worms and insects may all move through the (Fig. 9.24). Some of these soil types can be related to
soil mixing the layers and aerating it. the climatic conditions under which they form: geli-
Soils can be classified according to (Mack et al. sols indicate a cold climate whereas aridisols are
1993): characteristic of arid conditions, oxisols form most
. the degree of alteration (weathering) of the parent commonly under humid, tropical conditions and
material; vertisols form in subhumid to semi-arid climates
. the precipitation of soluble minerals such as calcite with pronounced seasonality. Particular hydrological
and gypsum; conditions are required for some soils, such as the

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) *

Fig. 9.24 Twelve major soil types
recognised by the US Soil Survey.