Soils
A soil is formed by physical, chemical and
biological processes that act on sediment, regolith or rock exposed at the land
surface (Retallack 2001). Collectively these soil-forming processes are known
as pedogenesis. Within a layer of sediment the
principal physical processes are the movement of water down from or up to the
surface and the formation of vertical cracks by the shrinkage of clays.
Chemical processes are closely associated with the vertical water movement as
they involve the transfer of dissolved material from one layer to another, the
formation of new minerals and the breakdown of some original mineral material.
The activity of plants is evident in most soils by the presence of roots and
the accumulation of decaying organic matter within the soil. The activity of
animals can have a considerable impact, as vertebrates, worms and insects may
all move through the soil mixing the layers and aerating it.
Soils can be classified according to (Mack et
al. 1993): .
the degree of alteration (weathering) of the parent material; .
the precipitation of soluble minerals such as calcite and gypsum;
oxidising/reducing
conditions (redox
conditions),
particularly with respect to iron minerals;
. the development of layering
(horizonation);
. the redistribution of
clays, iron and organic material into these different layers (illuviation);
. the amount of organic
matter that is preserved.
Twelve basic types of soil can be recognised
using the US Soil Survey taxonomy (Retallack 2001) (Fig. 9.24). Some of these
soil types can be related to the climatic conditions under which they form:
gelisols indicate a cold climate whereas aridisols are characteristic of arid
conditions, oxisols form most commonly under humid, tropical conditions and
vertisols form in subhumid to semi-arid climates with pronounced seasonality.
Particular hydrological conditions are required for some soils, such as the
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 immature 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 volcanic substrates.
Fig - Twelve major soil types
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.
calcrete soil profile are easily recognised in
palaeosols, so if the rate of development of a mature profile can be measured, the time over which an ancient profile formed can be estimated (Leeder 1975).
The passage of time can also be indicated by
other palaeosol types: entisols and inceptisols indicate that the time
available for soil formation on a particular surface was relatively short,
whereas other, more mature categories of palaeosol require a longer period of
exposure of the surface. These distinctions become useful when attempting to
assess rates of deposition on, for example, a floodplain surface: entisols
would indicate relatively rapid deposition, with little time for soil
development before flooding deposited more sediment on the surface, whereas a
well-developed spodosol, alfisol or ultisol suggests a much longer period of
time before the surface was covered with younger sediment. However, it should be
noted that the time taken for any soil profile to develop varies considerably
with temperature, rainfall and the availability of different minerals so time
estimates are always relative, not absolute. Also, soil profiles can become
complicated by the superimposition of a younger profile over an older one(Bown
& Kraus 1987).
Some types of modern and fossil soils have been given particular names. For example, seatearths are histisols, argillisols or spodosols that are common in the coal measures of northwestern Europe and North America (Percival 1986). They are characterised by a bed of organic matter underlain by a leached horizon of white sandstone from which iron has been washed out. Laterites are oxisols that are the product of extensive weathering of bedrock to form a soil that consists mainly of iron and aluminium oxides: examples of laterites may be found in the stratigraphic record as strongly reddened layers between basalt lava flows and provide evidence that the eruption was subaerial. Iron-rich oxisols that become cemented are known as ferricretes and they are a type of hardened soil profile called a duricrust. Duricrusts are highly resistant surfaces that develop over very long time periods (e.g. they are found associated with major unconformities; Retallack 2001); as well as iron-rich forms there are records of silcretes, which are silica-rich.
Identification of a palaeosol profile is
probably the most reliable indicator of a terrestrial environment. Channels are
not unique to the fluvial regime because they also occur in deltas, tidal
settings and deep marine environments, and thin sheets of sandstone are also
common to many other depositional settings. However, sometimes the recognition
of a palaeosol can be made difficult by diagenetic alteration (18.2), which can destroy the original
pedogenic features.
Posts
0 Comments