residual soil development
DESCRIPTION
Development of residual soil profileTRANSCRIPT
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CHAPTER 4
RESIDUAL TROPICAL SOIL DEVELOPMENT
Soil Development
The simplest definition of a soil is an unconsolidated layer of weathered rock which lies
upon bedrock and is a medium for plant growth (Bates and Jackson, 1984). Soil
scientists, such as Birkeland (1984), defines a soil as
“ . . . a natural body consisting of layers or horizons of mineral and/or organicconstituents of variable thickness which differ from the parent material in theirmorphological, physical, chemical, and mineralogical properties and theirbiological characteristics".
Soil Classification and Taxonomy
Soil scientists use physical features such as color, texture, and mineral composition to help
classify soils (Duchaufour, 1982; Birkeland, 1984; Jenny, 1994; Buol et al., 1997; Soil
Survey Staff, 1999). There are several soil classification systems in use today. Many of
these identifying features are not preserved as the soil lithifies into a paleosol. Geologists
such as Mack and James (1994) and Retallack (1988, 1994, 1997, 2001) have modified
the soil classification systems to classify paleosols based upon features preserved in the
lithified soils. Most paleosols, for example, are identified by features such as root traces,
soil horizons and soil structures (Retallack, 1988). Some soils and paleosols, such as
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residual soils, often lack fossils, root traces and other identifying features, and are
classified instead based upon mineral compositions.
Soils are classified based upon physical and chemical characteristics. There are two main
soils classification systems used: The Soil Survey Staff (1993, 1999) and FAO -
UNESCO (Food and Agriculture Organization of the United Nations) (1988). The soil
classification system used by the United States Soil Survey Staff is described in greater
detail below.
Duchaufour (1982) recognized that these classification systems do not take into account
the conditions necessary for soil formation, especially in regards to tropical / residual soils.
His system, while not widely used, is helpful in the classification of tropical soils such as
laterites. Table 4-1, below, shows the approximate relationship between Duchaufour,
FAO-UNESCO and USA-SSS’s classifications.
Soils are classified into orders, suborders, great groups, subgroups, and families based
upon the type of soil (i.e., mineral, organic, or both), the properties and characteristics of
specific horizons, color, and other physical characteristics (Soil Survey Staff, 1999). Each
soil order has a specific letter designation, as do the suborders, great groups, subgroups
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Duchaufour (1982)
FAO - UNESCO(Food and Agriculture
Organization of the UnitedNations, 1988)
USA(Soil Survey Staff, 1975, 1992)
fersiallitic soilscambisols, calcisols, luvisols,alisols
alfisols inceptisols
andosols andosols inceptisols
ferruginous soilsluvisols, alisols, lixisols,plinthosols
alfisols, ultisols
ferrisolsnitisols, acrisols, lixisols,luvisols, plinthosols
ultisols, oxisols
ferrallitic soils ferralsols, plinthosols oxisols
vertisols vertisols vertisols
podzols podzols spodosols
Table 4-1: World Soil Classification Systems
and families. These letter designations are not the same as those used for soil horizons.
Soil Horizons
Soils commonly contain layers, commonly called horizons by soil scientists. The Untied
States Department of Agriculture’s Soil Survey Division Staff Soil Survey Staff, 1993)
defines a soil horizon as follows:
“A soil horizon is a layer, approximately parallel to the surface of the soil,distinguishable from adjacent layers by a distinctive set of properties produced by
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the soil-forming processes. The term layer, rather than horizon, is used if all of theproperties are believed to be inherited from the parent material or no judgment ismade as to whether the layer is genetic.”
Each soil has it’s own “vertical distribution pattern” (Jenny, 1994), but nearly all have the
following basic profile:
• Horizon A: The Eluvial or leached horizon, from which most minerals and other
substances have been removed (Duchaufour, 1982; Jenny, 1994). Corresponds
with master soil horizon A (described below).
• Horizon B: The Illuvial horizon, in which the minerals and other substances
leached from horizon A accumulates (Duchaufour, 1982; Jenny, 1994).
Corresponds with master soil horizon B (described below).
• Horizon C: The parent rock. Corresponds with master soil horizons C and R
(described below).
There are two basic types of soil horizons: a genetic horizon and a diagnostic horizon. A
genetic horizon is used to “express a qualitative judgement about the kinds of changes that
are believed to have taken place in a soil” (Soil Survey Staff, 1999). There are 7 master
soil horizons and layers, represented by the capital letters O, A, E, B, C, R, and W. These
letters are combined with one or more of the 27 suffix symbols, represented by lower case
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letters, to describe a genetic soil horizon. Table 4-2 lists the master horizons and suffix
symbols used by soil scientists. These letter designations are not the same as, nor are they
interchangeable with, the letter designations used with the diagnostic soil horizons.
During the course of this field work, I found it convenient to give each of the paleosol
horizons a letter designation of A, B, C, and D. This was done for convenience, and does
not correspond with the master soil horizons and layers used by the Soil Survey Staff.
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Letter Description
O Horizons or layers composed primarily of organic matter. May or may not have beensaturated.
A Mineral horizons or layers formed at the surface or below an O horizon/layer. Nearlyall of the original rock structure has been destroyed.May exhibit either 1) and accumulation of humus mixed in with the mineral matter anddoes not have the characteristics of E or B horizons and/or 2) evidence of man-madeactivities such as cultivation, etc.
E A mineral horizon where the dominant feature is the absence of silicate clay, iron,aluminum or some combination of all of these minerals, resulting in a concentration ofsand and silt sized particles. Nearly all of the original rock structure has beendestroyed.
B Horizons formed below an A, E, or O horizon. Nearly all of the orginial rock structurehas been destroyed. Dominated by one or more of the following characteristics:1 Illuvial concentration of silicate clay, iron, aluminum, humus, carbonates, gypsum
and or silica2 Evidence of the removal or addition of carbonates3 Residual concentrations of oxides4 Sesquioxide coating which lowers the color value, raises the chroma, or becomes
redder in hue without the apparent illuviation of iron5 Alteration that either forms a silicate clay or liberates oxides. A granular, blocky or
prismatic structure is created if changes in volume occur as a result of a change inmoisture content.
6 Brittleness7 Strong gleying
C Sediment, saprolite or bedrock.
R Strongly cemented to indurated bedrock
W Water
Table 4-2: Master Soil Horizon and Layer Designations
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Residual Tropical Soils
Based on the information presented in the previous section, the paleosols of the Silverado
Formation are likely residual soils. Consequently, a discussion of residual soils is in order.
Formation of tropical residual soil requires the following: physical and chemical
weathering, the leaching of insoluble materials and the accumulation of insoluble residues,
and the movement of fine particles downward (lessivage). Physiochemical mechanism is a
type of hydrolysis that occurs only in tropical soils (Duchaufour, 1982). This type of
weathering occurs as a function of neutral to slightly acidic hydrolysis, and is generally not
influenced by surface organic matter (Duchaufour, 1982; Fookes, 1997). Kaolinites,
bauxites and laterites all are examples of residual soils.
Kaolinites
The word “kaolinite” has two basic meanings. First, it refers to a high-aluminum clay
mineral belonging to the kaolin group. It is typically a secondary mineral formed via
weathering or hydrothermal alteration of aluminum silicates (Klein and Hurlbut, 1985).
The term can also refer to a type of residual soil formed in tropical environments and
nearly constant rainfall. The lack of a dry season prohibits the development of iron oxides,
leaving a residual soil of kaolinite and possibly quartz.
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Laterites
There is no consensus as to what defines a laterite, nor is there agreement as to how they
are formed (Duchaufour, 1982; Jenny, 1994; Retallack, 1997). The U.S. Soil Survey Staff
(1993) defines a laterite as a plinthite, an hematite-rich mottled red/yellow and white clay
zone; others consider only the hard, hematite- and clay-rich surface crust (also called a
cuirasse) a laterite. Jenny (1994) bases his definition of a laterite profile on that of
Harrassowitz, which is comprised of the following: humus soil (may not be present); iron
crust; accumulation zone (sesquioxides - kaolinite, etc.); decomposition zone (saprolite);
and finally, fresh rock. Retallack (1997) prefers to define a laterite as a rock or part of a
soil, not a true soil. His laterite profile, from top to bottom, is similar to that described by
Jenny (1994): a red soil "cap"; the laterite, a red, hematite-rich clay zone lacking humus,
bases and silica; a "pallid zone" or a white clay zone that may or may not contain mottles;
a "white china clay"; the saprolite; the parent rock (Retallack, 1997). Duchaufour (1982)
and Fookes (1997) avoid the term "laterite" altogether, preferring Duchaufour's system of
classification for tropical soils.
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Retallack (1997) describes four basic theories on laterite development, included below:
Residuum
Laterites which develop in altered bedrock left after an long period of weathering.
The major problem with this theory is that it requires a large amount of rock to
produce enough iron for hematite or goethite to form.
Soil Horizon
In this theory, laterites develop as a precipitate either at the edge of capillary rise or
just above a fluctuating water table. This theory may account for the development of
some laterites, but it does not account for the development of laterites up to several
meters thick.
Deposit
Laterites as a depositional feature are considered to be the origin of lateritic breccias
and loose, pea-sized pisolitic aggregates. While this does appear to be valid for these
types of laterites, the theory does not explain massive laterites that grade into pallid or
kaolinitic zones and then into a saprolite.
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Groundwater Precipitate
Acidic groundwater from marshes and swamps leaches the bedrock well below the
influence of surface interactions. This theory accounts for the development of
kaolinite and hematite-rich zones, and can be used to explain the variable thicknesses
seen in some deposits, but it does not satisfactorily explain the origin of all laterites.
Both Duchaufour (1982) and Fookes (1997) favor this residual soil origin for laterites,
which appears to be similar to this theory, although they do not require a marsh or
swamp environment to begin the leaching process.
Duchaufour (1982) has determined that there are three phases of the weathering cycle for
tropical environments: fersiallitisation, where 2:1 clays (micas, chlorite) are dominant;
ferrugination, characterized by kaolinite and 2:1 clays (smectite, illite); and finally
ferrallitisation, dominated by kaolinite and gibbsite. These phases are characterized by an
increase in weathering of the primary minerals, and increase in the loss of combined silica,
and an increase in the amount of clays formed from the alteration of the primary minerals
(Duchaufour, 1982).
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Fersiallitisation
Weathering of primary minerals produces primarily 2:1 clays. There is also
considerable amounts of free iron oxides formed and the soils becomes rubified. An
argillic (Bt) horizon can develop as well. Fersiallitic soils typically occur in subtropical
or Mediterranean climates with a dry season.
Ferrugination
Weathering increases, although primary minerals such as orthoclase and muscovite can
still be present. There is an increase in the amount of newly formed 1:1 clays such as
kaolinite, especially in relation to 2:1 clays. Rubification may or may not occur.
These soils commonly form in humid subtropical environments or humid tropical
regions with a dry season.
Ferrallitisation
All minerals, except for quartz, are completely weathered and altered to kaolinite. The
entire profile has been reduced to quartz, kaolinite or gibbsite, and hematite or
goethite. No true argillic horizon forms. Hot, humid tropical regions typically
produce these types of soils, although they can be found in drier climates such as semi-
rainforests and in savannas.
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Ferrallitic soils typically form a profile comprised of three zones: The upper zone, the
middle zone, and a zone of deep weathering (saprolite) (Duchaufour, 1982).
Upper Zone
Comprised of A and B horizons. The A horizon contains a great deal of plant debris
which is rapidly decomposed, and is highly depleted in clay and iron. In contrast, the
B horizon is typically enriched in iron, producing either an ochreous (gibbsite), red
(hematite), or mottled red and white color in the soil.
Middle Zone
This zone is characterized by large, irregularly shaped red or ochreous mottles and is
typically several meters thick. If Horizon B contains mottles, they extend down into
the Middle Zone, although there is less contrast between the mottles and the overall
color is lighter.
Deeply Weathered Soil / Saprolite
Saprolites are the transition zone between the residual soil and the parent rock.
Weathering is often irregular, with minerals in various phases of alteration.
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Classification of Paleosol 1
Laterites and other residual soils are difficult to classify using traditional classification
systems (Duchaufour, 1982), as they lack horizons as defined by the Soil Survey Staff
(1999) and other classification systems. Therefore, Duchaufour’s classification system
(1982), having been created for the express purpose of classifying residual soils, best suits
classifying Paleosol 1.
High precipitation rates found in humid tropical environments provide a nearly constant
influx of water, and all minerals except quartz are leached out. The remaining Al and Si
ions precipitate out as aluminum hydroxides, and further alter to kaolinite/gibbsite (Garrels
and Christ, 1965; James et al., 1981; Duchaufour, 1982; Mack, et al., 1993; Jenny, 1994;
Buol, et al., 1997; Fookes, 1997). Kaolinite will form from gibbsite when there is an
excess of Si(OH)4 in solution and in acid conditions, especially in granitic or quartz-rich
sedimentary rocks (Duchaufour, 1982; Jenny, 1994;Buol et al., 1997). Iron and
aluminum oxides tend to remain in situ; hematite forms where the soil is subject to
seasonal dry periods (Buol et al., 1997). Poor drainage within this kaolinitic horizon
produces red and white mottling. Fluctuation of watertable levels promotes the
development of a ferricrete crust (Duchaufour, 1982; Fookes, 1997).
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Figure 4-1 graphically compares a “typical” laterite profile as described by Jenny (1994)
and Retallack (1997) with Paleosol 1, Horizon A in particular. Horizon A is comprised of
an iron crust (Retallack’s laterite, Fookes’ (1994) ferricrete, and Duchaufour’s Upper
Zone), and a mottled zone and a pallid zone (Retallack’s pallid zone and Duchaufour’s
Middle Zone). Thus, Horizon A could be classified as a ferrallitic soil using
Duchaufour’s system based upon the presence of quartz, kaolinite and hematite and the
lack of other minerals.
Horizon B, similar in composition to the mottled zone of Horizon A, may be a portion of a
laterite as described by Duchaufour (1982), Jenny (1994) and Retallack (1997). The red
and white mottling of the kaolinite in addition to the etched quartz fits the description of
Retallack’s pallid zone and Duchaufour’s Middle Zone. The lack of the iron
crust/ferricrete capping the mottled zone may be because the development of the laterite
was incomplete, or, as Duchaufour (1982) and Fookes (1997) suggest, water-table levels
during formation of the horizon did not fluctuate enough to produce the ferricrete crust.
A kaolinite residual soil forms under similar conditions as does a laterite: intense
weathering and leaching due to high precipitation rates alters all minerals (except quartz)
to kaolinite (Garrels and Christ, 1965; James et al., 1981; Duchaufour, 1982; Mack et al.,
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Figure 4-1: Comparison between a laterite and Paleosol 1. Laterite characteristics andgeneralized profile from Retallack (1997).
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1993; Jenny, 1994; Buol, et al., 1997; Fookes, 1997). Horizons C and D resemble this
description of a kaolinitic residual soil, being comprised of kaolinite and quartz.
In summary, Paleosol 1 contains several characteristics of a residual soil: kaolinite and
quartz composition, etching and dissolution of quartz grains, and FeO mottling, to name a
few. As such, the paleosols of the Silverado Formation probably formed under subtropical
to tropical conditions with high precipitation rates and possibly a short, hot, dry season.