418
Principal Soil Types
The most important types of soils are summarized in
Fig. 9.2. If the elimate is warm and annual precipitation, P, lower than potential evaporation, E, from the
soil surface, vegetation is sparse or absent and infiltrated water can rarely percolate the underground.
Dissolved constituents from the soil zone are
reprecipitated elose to the land surface as gypcretes,
calcretes, and possibly silcretes as a result of water
loss to evaporation. Calcium sulfate for gypsum may
be provided either by ascending water from a shallow
water table or by oxidation of pyrite present in the
parent rock. Calcium carbonate for calcrete formed
elose to the surface normally comes from the soil
zone or from eolian dust. Only in alluvial plains and
playas does Ca 2 + (and So/) from a shallow groundwater table contribute to the formation of calcrete or
gypsum nodules.
Where annual precipitation is higher than evaporation (P>E), the vegetation cover becomes dense and
infiltrated water and part of its solutes can reach the
groundwater table. This process also allows the
downward displacement of elay partieles (lessivage)
and some particulate organic matter and their enrichment in deeper horizons of the soil profile. Under
cool and temperate conditions, organic litter decays
slowly and causes a reduced grey upper soil horizon
with acidic soil water transporting Fe2+ downward
(podsols and gleysols) or, in places, laterally. Warm
elimate favors rapid decay of the topsoil organic
layer and thus leads to soil water of higher pH values
allowing dissolution of both F 2 + and Si0 2 from various minerals. This process generates ferralsols,
oxisols, and laterites. As soon as oxic or partially
oxic conditions are established (especially in the dry
season), iron and some other constituents are
reprecipitated from the downward percolating soil
water. The resulting Fe3+ compounds (later transformed into hematite) form small concretions
(illuvial horizon) or, in particular under peat
(histosols), pyrite and siderite concretions. In the
case of ferralsols, oxisols and laterites, dissolved silica leaves the soil zone and Fe 3 + and Al compounds
are enriched and may form ferricrusts (laterite)
andlor bauxite. Pedogenic silcretes develop in a similar way as calcretes, but apparently in a somewhat
wetter elimate with seasonal rainfall. When primary
minerals such as feldspar are already destroyed, silica
is mainly provided by elay minerals, present in the
soil or saprolite, and reprecipitated as microcrystalline quartz. Aluminum should either be fixed in stable minerals or be more soluble than silica within the
zone of silicification. A kind of columnar or lenticular structure appears to be characteristic of silcrete.
Thickness and specific structures of calcrete,
ferricrete, and silcrete depend on the time period
available for their development (Fig. 9.3a and b).
Chapter 9 Denudation and Mass Transport
Thick representatives of these "duricrusts" or hardpans need time periods of hundreds of thousands up
to millions ofyears to form. For such long time spans
it is not uncommon that certain land surfaces have
experienced c1imate change which has produced different duricrusts on top of each other. When the elimate became slowly dryer, lateritic crusts may have
been overprinted at places by silcretes and these in
turn by calcretes (Fig. 9.2).
All these pedogenic duricrusts have counterparts
formed many meters below the land surface within
the capillary fringe of a fluctuating groundwater table. These are "groundwater calcretes", "groundwater laterites", and "groundwater silcretes" (Fig. 9.2)
which more or less originate from the early diagenetic cementation of unconsolidated sediments, e.g.
channel fills. The cement material normally comes
from chemical weathering (e.g. the saprolite) and is
transported laterally by groundwater flow. Reprecipitation takes place in the water-unsaturated zone
above the groundwater table. Groundwater with a
high Mg/Ca ratio of dissolved species (e.g. elose to a
playa system) can generate groundwater dolocrete.
There are some distinctive features to discriminate
between pedogenic calcretes or silcretes generated
within the soil zone and diagenetic "crusts" related to
the groundwater table (Wright 1994 and 1996).
Silcretes consisting mainly of opal and chalcedony
seem to have been precipitated from groundwater
richer in dissolved silica (>6 mg/I) than meteoric soil
water.
Vertisols develop on substrates rich in swelling
elay minerals, such as smectites or mixed-layer minerals, in a seasonal dry and wet elimate. These soils
and their paleosols exhibit structures associated with
shrinking (desiccation cracks) and swelling (pseudoantidunes and slickensides; Fig. 9.2). Below the zone
of plastic deformation, carbonate nodules can occur.
Vertisols are common in ancient continental
elaystones and marls.
Soils Controlled by Sediment Aggradation or
Erosion
In environments with regular or episodic sediment
accumulation, e.g. on flood plains of meandering
rivers, the time available to develop soils at the land
surface is limited. The higher the sedimentation rate,
SR, the shorter is the so-called residence time, RT, of
the soil to stay within the near-surface zone of soil
formation. This relationship is shown in Fig. 9.3a for
a certain depth of active pedogenesis, H ap (Wright
1992 and 1994). For example, when H ap =2 m and
SR=2 mm/a, the residence time of the soil is
RT=1000 a. As shown in Fig. 9.3b, this is the minimum time span to develop some initial soil features
such as destratification, carbonate nodules, or argillic
Principal Soil Types
The most important types of soils are summarized in
Fig. 9.2. If the elimate is warm and annual precipitation, P, lower than potential evaporation, E, from the
soil surface, vegetation is sparse or absent and infiltrated water can rarely percolate the underground.
Dissolved constituents from the soil zone are
reprecipitated elose to the land surface as gypcretes,
calcretes, and possibly silcretes as a result of water
loss to evaporation. Calcium sulfate for gypsum may
be provided either by ascending water from a shallow
water table or by oxidation of pyrite present in the
parent rock. Calcium carbonate for calcrete formed
elose to the surface normally comes from the soil
zone or from eolian dust. Only in alluvial plains and
playas does Ca 2 + (and So/) from a shallow groundwater table contribute to the formation of calcrete or
gypsum nodules.
Where annual precipitation is higher than evaporation (P>E), the vegetation cover becomes dense and
infiltrated water and part of its solutes can reach the
groundwater table. This process also allows the
downward displacement of elay partieles (lessivage)
and some particulate organic matter and their enrichment in deeper horizons of the soil profile. Under
cool and temperate conditions, organic litter decays
slowly and causes a reduced grey upper soil horizon
with acidic soil water transporting Fe2+ downward
(podsols and gleysols) or, in places, laterally. Warm
elimate favors rapid decay of the topsoil organic
layer and thus leads to soil water of higher pH values
allowing dissolution of both F 2 + and Si0 2 from various minerals. This process generates ferralsols,
oxisols, and laterites. As soon as oxic or partially
oxic conditions are established (especially in the dry
season), iron and some other constituents are
reprecipitated from the downward percolating soil
water. The resulting Fe3+ compounds (later transformed into hematite) form small concretions
(illuvial horizon) or, in particular under peat
(histosols), pyrite and siderite concretions. In the
case of ferralsols, oxisols and laterites, dissolved silica leaves the soil zone and Fe 3 + and Al compounds
are enriched and may form ferricrusts (laterite)
andlor bauxite. Pedogenic silcretes develop in a similar way as calcretes, but apparently in a somewhat
wetter elimate with seasonal rainfall. When primary
minerals such as feldspar are already destroyed, silica
is mainly provided by elay minerals, present in the
soil or saprolite, and reprecipitated as microcrystalline quartz. Aluminum should either be fixed in stable minerals or be more soluble than silica within the
zone of silicification. A kind of columnar or lenticular structure appears to be characteristic of silcrete.
Thickness and specific structures of calcrete,
ferricrete, and silcrete depend on the time period
available for their development (Fig. 9.3a and b).
Chapter 9 Denudation and Mass Transport
Thick representatives of these "duricrusts" or hardpans need time periods of hundreds of thousands up
to millions ofyears to form. For such long time spans
it is not uncommon that certain land surfaces have
experienced c1imate change which has produced different duricrusts on top of each other. When the elimate became slowly dryer, lateritic crusts may have
been overprinted at places by silcretes and these in
turn by calcretes (Fig. 9.2).
All these pedogenic duricrusts have counterparts
formed many meters below the land surface within
the capillary fringe of a fluctuating groundwater table. These are "groundwater calcretes", "groundwater laterites", and "groundwater silcretes" (Fig. 9.2)
which more or less originate from the early diagenetic cementation of unconsolidated sediments, e.g.
channel fills. The cement material normally comes
from chemical weathering (e.g. the saprolite) and is
transported laterally by groundwater flow. Reprecipitation takes place in the water-unsaturated zone
above the groundwater table. Groundwater with a
high Mg/Ca ratio of dissolved species (e.g. elose to a
playa system) can generate groundwater dolocrete.
There are some distinctive features to discriminate
between pedogenic calcretes or silcretes generated
within the soil zone and diagenetic "crusts" related to
the groundwater table (Wright 1994 and 1996).
Silcretes consisting mainly of opal and chalcedony
seem to have been precipitated from groundwater
richer in dissolved silica (>6 mg/I) than meteoric soil
water.
Vertisols develop on substrates rich in swelling
elay minerals, such as smectites or mixed-layer minerals, in a seasonal dry and wet elimate. These soils
and their paleosols exhibit structures associated with
shrinking (desiccation cracks) and swelling (pseudoantidunes and slickensides; Fig. 9.2). Below the zone
of plastic deformation, carbonate nodules can occur.
Vertisols are common in ancient continental
elaystones and marls.
Soils Controlled by Sediment Aggradation or
Erosion
In environments with regular or episodic sediment
accumulation, e.g. on flood plains of meandering
rivers, the time available to develop soils at the land
surface is limited. The higher the sedimentation rate,
SR, the shorter is the so-called residence time, RT, of
the soil to stay within the near-surface zone of soil
formation. This relationship is shown in Fig. 9.3a for
a certain depth of active pedogenesis, H ap (Wright
1992 and 1994). For example, when H ap =2 m and
SR=2 mm/a, the residence time of the soil is
RT=1000 a. As shown in Fig. 9.3b, this is the minimum time span to develop some initial soil features
such as destratification, carbonate nodules, or argillic