422
Ganges, Yangtze, Yellow and Parami rivers, and in
several major rivers of Europe (Konta 1988).
Chlorite is a major constituent of slates and greenschists. If it occurs in the river load in considerable
quantities it comes from the mechanical erosion of
pre-existing rocks containing this mineral. Chlorite is
therefore a main constituent (around 20% ofthe total
suspended load) of rnany rivers draining high mountain ranges, such as the Hirnalayan rivers or some
rivers in North and South America (Mackenzie, St.
Lawrence, Orinoco). However, chlorite is destroyed
in warm, humid climates, because it is very sensitive
to chemical weathering. Hence, the presence of detrital chlorite indicates either cold or possibly arid climate (see also, e.g., Chamley 1989).
Climate Control of Soil-Derived Minerals
If transport-limited conditions prevail in large parts
of the drainage area, thick soil profiles can develop.
Then slow mechanical denudation mainly removes
soils but little parent rock material. As a result, soilformed neo-minerals tend to predominate in the suspended river load. Under these conditions it is difficult to find out the original nature of the source
rocks.
In humid tropical and subtropical regions, a relatively short time period (often some thousands of
years) is needed to completely remove carbonate in
solution from the soil and saprolite. Then, during the
first stage of mineral transformation, kaolinite is the
predominating, newly formed clay mineral reflecting
the clirnate rather than the mineralogical composition
of the source rocks. Iron-oxyhydrate and Fe Z 0 3 (hematite) lead to red soils (laterites, see above and
Chap. 6.3). In a second phase of mineral transformation, even the common clay minerals (aluminum silicates, including kaolinite) become unstable and their
silica content is dissolved and carried away by water.
Hydroxides and oxides of a1uminum and iron are left
behind as a residue and may form thick lateritic
crusts (ferricrusts, Figs. 9.1a and 9.2) or, after some
lateral displacement, locally accumulate to bauxite
deposits (e.g. Ollier 1969; Chorley et al. 1984;
Retallack 1990; Valeton in Füchtbauer 1988).
In temperate humid zones, carbonate is still well
dissolved from the soi1 zone and underlying rocks.
Here, illite is a characteristic, low solubility weathering product. It is on1y under semi-arid conditions that
carbonate rernains in the soil zone as reprecipitated
calcrete or caliche (cf. Fig. 9.1a and 9.2).
Generally, the neo-mineral group includes minerals of the montrnorillonite/smectite types (mostly basic parent rocks, temperate to subtropical climates),
the sheet-silicates of the illite/muscovite type (humid
temperate climate), kaolinite and gibbsite (and the
iron oxide goethite; semi-arid to tropical climates).
Chapter 9 Denudation and Mass Transport
The iron oxides indicate either humid conditions
(goethite) or a semi-arid to subtropical climate (hematite).
Montmorillonite is an important constituent in the Caroni
and some other tributaries ofthe Amazon River (crystalline
rocks, tropical c\imate), in the Orange River (SW Africa),
and in the Deccan Plateau rivers of India draining basaltic
rocks and their weathering products (subtropical c\imate).
It is also common in the Mississippi River near its mouth,
where the :> 1 0 Ilm size fraction contains 40% smectite or
montmorillonite, 25% illite, and 20% kaolinite (Potter et
al. 1975). Kaolinite and gibbsite are abundant in the soils
of wet tropical c\imates, where the less stable chlorite disappears. Kaolinite is a principal component in the river
loads ofthe Orinoco and Caroni, Niger, Congo, the Deccan
rivers, arid in some rivers of New Guinea; gibbsite was
reported only from the Niger River.
It is a general rule that small river systems displaya
more uniform mineralogical composition of their suspended solids than large systems. The latter normally
reflect the influence of mixed parent rocks, different
relief, clirnatic zones, and soils.
There are more soi1-formed clay minerals than
those mentioned here, but a more detailed treatment
of this topic is beyond the scope of this text (see e.g.
Chamley 1989; Weaver 1989; Velde 1995). In any
case, one should be cautious in deducing the
paleoclimate of a specific region solely from the clay
mineral composition of sediments derived from the
area.
Chemical-mineralogical weathering is also characterized by
different partitioning of elements between the dissolved
and the suspended phases. Generally, the solids tend to
become enrlched in aluminum, iron, and (in regions of
humid, temperate c\imate) in silicon, while sodium, calcium, and (to a lesser extent) potassium and magnesium are
carried away in solution (e.g., Stallard 1985). Ifthe cationpoor minerals (kaolinite, gibbsite, goethite) are regarded as
the end products of intensive weathering, then a kind of
chemical-mineralogical maturity of the soil-derived solids
can be defined (Emeis 1985; Konta 1988) with the aid of
their bulk chemical composition. Relatively high concentrations of A1 3 + (and ferric iron), or relatively low Si02 contents and low concentrations of Na + and Ca 2 + (and Mg22+,
K+) indicate an advanced maturity (tropical rivers). Low
maturity, expressed by a low Al/Si ratio, is typical of arid
drainage basins. With growing maturity, the Al/Si ratio
also increases.
Sediment Recycling
In the early history of the Earth, the land masses
were entirely or predominantly composed of igneous
rocks and experienced jirst-cycle weathering. As
long as soil-generating processes and hence the formation of neo-minerals were limited, in particular
hill-slope erosion and valley cutting could provide
unstable rock-derived minerals for sediments.
Ganges, Yangtze, Yellow and Parami rivers, and in
several major rivers of Europe (Konta 1988).
Chlorite is a major constituent of slates and greenschists. If it occurs in the river load in considerable
quantities it comes from the mechanical erosion of
pre-existing rocks containing this mineral. Chlorite is
therefore a main constituent (around 20% ofthe total
suspended load) of rnany rivers draining high mountain ranges, such as the Hirnalayan rivers or some
rivers in North and South America (Mackenzie, St.
Lawrence, Orinoco). However, chlorite is destroyed
in warm, humid climates, because it is very sensitive
to chemical weathering. Hence, the presence of detrital chlorite indicates either cold or possibly arid climate (see also, e.g., Chamley 1989).
Climate Control of Soil-Derived Minerals
If transport-limited conditions prevail in large parts
of the drainage area, thick soil profiles can develop.
Then slow mechanical denudation mainly removes
soils but little parent rock material. As a result, soilformed neo-minerals tend to predominate in the suspended river load. Under these conditions it is difficult to find out the original nature of the source
rocks.
In humid tropical and subtropical regions, a relatively short time period (often some thousands of
years) is needed to completely remove carbonate in
solution from the soil and saprolite. Then, during the
first stage of mineral transformation, kaolinite is the
predominating, newly formed clay mineral reflecting
the clirnate rather than the mineralogical composition
of the source rocks. Iron-oxyhydrate and Fe Z 0 3 (hematite) lead to red soils (laterites, see above and
Chap. 6.3). In a second phase of mineral transformation, even the common clay minerals (aluminum silicates, including kaolinite) become unstable and their
silica content is dissolved and carried away by water.
Hydroxides and oxides of a1uminum and iron are left
behind as a residue and may form thick lateritic
crusts (ferricrusts, Figs. 9.1a and 9.2) or, after some
lateral displacement, locally accumulate to bauxite
deposits (e.g. Ollier 1969; Chorley et al. 1984;
Retallack 1990; Valeton in Füchtbauer 1988).
In temperate humid zones, carbonate is still well
dissolved from the soi1 zone and underlying rocks.
Here, illite is a characteristic, low solubility weathering product. It is on1y under semi-arid conditions that
carbonate rernains in the soil zone as reprecipitated
calcrete or caliche (cf. Fig. 9.1a and 9.2).
Generally, the neo-mineral group includes minerals of the montrnorillonite/smectite types (mostly basic parent rocks, temperate to subtropical climates),
the sheet-silicates of the illite/muscovite type (humid
temperate climate), kaolinite and gibbsite (and the
iron oxide goethite; semi-arid to tropical climates).
Chapter 9 Denudation and Mass Transport
The iron oxides indicate either humid conditions
(goethite) or a semi-arid to subtropical climate (hematite).
Montmorillonite is an important constituent in the Caroni
and some other tributaries ofthe Amazon River (crystalline
rocks, tropical c\imate), in the Orange River (SW Africa),
and in the Deccan Plateau rivers of India draining basaltic
rocks and their weathering products (subtropical c\imate).
It is also common in the Mississippi River near its mouth,
where the :> 1 0 Ilm size fraction contains 40% smectite or
montmorillonite, 25% illite, and 20% kaolinite (Potter et
al. 1975). Kaolinite and gibbsite are abundant in the soils
of wet tropical c\imates, where the less stable chlorite disappears. Kaolinite is a principal component in the river
loads ofthe Orinoco and Caroni, Niger, Congo, the Deccan
rivers, arid in some rivers of New Guinea; gibbsite was
reported only from the Niger River.
It is a general rule that small river systems displaya
more uniform mineralogical composition of their suspended solids than large systems. The latter normally
reflect the influence of mixed parent rocks, different
relief, clirnatic zones, and soils.
There are more soi1-formed clay minerals than
those mentioned here, but a more detailed treatment
of this topic is beyond the scope of this text (see e.g.
Chamley 1989; Weaver 1989; Velde 1995). In any
case, one should be cautious in deducing the
paleoclimate of a specific region solely from the clay
mineral composition of sediments derived from the
area.
Chemical-mineralogical weathering is also characterized by
different partitioning of elements between the dissolved
and the suspended phases. Generally, the solids tend to
become enrlched in aluminum, iron, and (in regions of
humid, temperate c\imate) in silicon, while sodium, calcium, and (to a lesser extent) potassium and magnesium are
carried away in solution (e.g., Stallard 1985). Ifthe cationpoor minerals (kaolinite, gibbsite, goethite) are regarded as
the end products of intensive weathering, then a kind of
chemical-mineralogical maturity of the soil-derived solids
can be defined (Emeis 1985; Konta 1988) with the aid of
their bulk chemical composition. Relatively high concentrations of A1 3 + (and ferric iron), or relatively low Si02 contents and low concentrations of Na + and Ca 2 + (and Mg22+,
K+) indicate an advanced maturity (tropical rivers). Low
maturity, expressed by a low Al/Si ratio, is typical of arid
drainage basins. With growing maturity, the Al/Si ratio
also increases.
Sediment Recycling
In the early history of the Earth, the land masses
were entirely or predominantly composed of igneous
rocks and experienced jirst-cycle weathering. As
long as soil-generating processes and hence the formation of neo-minerals were limited, in particular
hill-slope erosion and valley cutting could provide
unstable rock-derived minerals for sediments.