Page 161 - Sedimentology and Stratigraphy

148 Rivers and Alluvial Fans

waterlogged setting that histosols (peaty soils) form
in. Other types are indicative of the degree of the
maturity of the soil profile (and hence the time over
which the soil has developed); entisols are very imma-

ture and inceptisols show more development, but are
less mature than the other types lower in the list. The
type of vegetation is an important factor in some
cases: spodosols, alfisols and ultisols are soils formed
in forests, whereas mollisols are grassland soils.
Finally, the formation of andisols is restricted to vol-
canic substrates.

9.7.2 Palaeosols

A palaeosol is a fossil soil. Many of the characteristics
of modern soils noted above can be recognised in soils
that formed in the geological past (Mack et al. 1993;
Retallack 2001). These features include the presence

*
of fossilised roots, the burrows of soil-modifying
organisms, vertical cracks in the sediment and layers
enriched or depleted in certain minerals. The study of
palaeosols provides important information about
ancient landscapes and in particular they can indicate
the palaeoclimate, the type of vegetation growing and
the time period during which a land surface was

*
exposed.

The precipitation of calcium carbonate within the

soil is a conspicuous feature of some aridisols that form
in semi-arid to arid climates. These calcrete soils form
Fig. 9.25 A calcrete forms by precipitation of calcium
by the movement of water through the soil profile carbonate within a soil in an arid or semi-arid environment.
precipitating calcium carbonate as root encrustations
(rhizocretions) and as small soil nodules (glaebules)
(Wright & Tucker 1991). The nodules grow and co-
alesce as precipitation continues to form a fully devel-
oped calcrete, which consists of a dense layer of of exposure of the surface. These distinctions become
calcium carbonate near to the surface with tepee useful when attempting to assess rates of deposition
structures, i.e. domes in the layer formed by the on, for example, a floodplain surface: entisols would
expansion of the calcium carbonate as it is precipi- indicate relatively rapid deposition, with little time for
tated (Allen 1974) (Fig. 9.25). The stages in the soil development before flooding deposited more sedi-
development of a calcrete soil profile are easily recog- ment on the surface, whereas a well-developed spodo-
nised in palaeosols, so if the rate of development of a sol, alfisol or ultisol suggests a much longer period of
mature profile can be measured, the time over which time before the surface was covered with younger
an ancient profile formed can be estimated (Leeder sediment. However, it should be noted that the time
1975). taken for any soil profile to develop varies consider-
The passage of time can also be indicated by other ably with temperature, rainfall and the availability of
palaeosol types: entisols and inceptisols indicate that different minerals so time estimates are always rela-
the time available for soil formation on a particular tive, not absolute. Also, soil profiles can become com-
surface was relatively short, whereas other, more plicated by the superimposition of a younger profile
mature categories of palaeosol require a longer period over an older one (Bown & Kraus 1987).