416
agents, but groundwater circulation, ice and wind
transport also playa significant part in many regions.
Mass transfer to sedimentary basins may take place
discontinuously due to intermediate storage of loose
material in valleys, on flood plains, or in lakes upstream of the basin considered.
Disintegration of rocks due to stress release, as
weIl as thermal expansion and contraction is apparent
even at a depth of several tens of meters below the
land surface (Fig. 9.lb). Newly formed joints near
the surface and increasing pore space favor the circulation of water and thus accelerate mechanical and
chemical weathering. The zone of most pronounced
weathering is near the surface in the pedosphere (Fig.
9.la), where biological activity accelerates the alteration of rocks and minerals. Significant weathering
also occurs below the pedosphere, in the so-called
saprolite (cf. Fig. 9.3). Then the base ofthe saprolite
may represent the most active front of weathering.
The term regolith is mostly used in a broader sense; it
signifies the mantle of unconsolidated and fragmental rock material covering the bedrock whether
formed in place or transported.
In general, large, thermodynamically unstable
minerals are transformed into smaIler, stable components such as clay minerals (neo-minerals, "mineralogieal" weathering). At the same time, the. parent
minerals lose part of their solid matter by chemical
reactions and dissolution ("chemieal" weathering).
Thus, the entire alteration process should be referred
to as "biochemical-chemical-mineralogical" weathering, but it is frequently called chemical weathering.
Several minerals are completely dissolved, such as
carbonate minerals in humid regions.
Chemical-mineralogical and biological weathering
are dominant in wet climates, whereas in arid climates rocks are disintegrated mainly by mechanical
processes. The products of chemical weathering are
best developed in lowland areas or on flat plateaus,
where the altered material of the saprolite cannot be
removed readily from its parent rock. Hence, deeply
weathered profiles of completely altered parent rock
are common in tropical lowlands or on large
nondissected plateaus (Figs. 9.la and 9.3d). Regions
of high topographie relief in conjunction with substantial rainfall are most important in delivering particulate matter into rivers.
Erosion on hillslopes is controlled by both the
weathering rate and transport processes which remove the disintegrated, loose material at the surface.
Erosion may be "weathering-limited', if the capacity
of the transport processes exceeds the rate at which
erodible material is generated by weathering (cf. Fig.
9.3c and d). As a result, the bedrock is exposed and
the slopes tend to steepen, thus further promoting
mechanical denudation. In contrast, erosion is
"transport-limited' if more material can be supplied
by weathering than can be removed by transport proChapter 9 Denudation and Mass Transport
cesses. In this case, thick soil profiles develop, and
transport of materials in solution predominates.
The principal relationship between climate and
weathering products of the source area with, e.g.,
granitic bedrock, providing both clastic material and
dissolved river load for a sedimentary basin, is summarized in Fig. 9.lc. Tropical regions tend to deliver
predominantly neo-minerals and a small portion of
the most stable minerals from pre-existing bedrocks
as terrigenous sediment components. Temperate, humid regions supply both neo-minerals and stable preexisting minerals, whereas arid regions deli ver comparatively unstable minerals. This simplified model
may be modified by grain sorting during sediment
transport. Fine-grained suspended river load moves
much faster and usually over longer distances than
the coarser, sandy and gravelly bed load (Fig. 9.1b).
It is therefore usually separated from the bed load.
The dissolved river loads in the three climatic
zones also differ markedly both in the concentrations
of various chemical species and in their freight rates
(Fig. 9.lc). Due to intense precipitation and chemical
weathering, more solid matter per unit area and time
can be dissolved in the wet tropics than in other
zones, provided the bedrock is not protected by thick,
clay-rich vegetated soil covers. In arid regions, mechanical processes are more important than dissolution. These and some further aspects will be further
discussed quantitatively in Sections 9.2 and 9.3.
9.1.2 Modern Soils and Paleosols (Overview)
Introduction
Soils play an important part not only in providing
various erodible materials for sediment transport and
basin filling, but also as indicators of paleoclimate
and depositional or erosional processes which have
been operating in the past. Paleosols have also become a significant tool in continental sequence stratigraphy (cf. Chap. 7.6). However, the genesis of
modem soils (pedogenesis) and older buried soils
(i.e. paleosols) and the degree of their preservation,
partial destruction, or diagenetic overprint are complex processes. The following brief overview mainly
stresses sedimentological aspects.
Soil forming processes, soil classification, and paleosols
are treated in several special publications and books (e.g.
Wright 1986; Reinhardt and Sigleo 1988; Retallack 1990;
Martini and Chesworth 1992). Duricrusts are described e.g.
by Wopfner (1978), Twidale and Milnes (1983), Milnes
and Thiry (1992).
Soils develop at the land surface under different climates, hydrological regimes, vegetation and other
biological activities. They are further influenced by
the nature of the parent rocks or unconsolidated sedi-
agents, but groundwater circulation, ice and wind
transport also playa significant part in many regions.
Mass transfer to sedimentary basins may take place
discontinuously due to intermediate storage of loose
material in valleys, on flood plains, or in lakes upstream of the basin considered.
Disintegration of rocks due to stress release, as
weIl as thermal expansion and contraction is apparent
even at a depth of several tens of meters below the
land surface (Fig. 9.lb). Newly formed joints near
the surface and increasing pore space favor the circulation of water and thus accelerate mechanical and
chemical weathering. The zone of most pronounced
weathering is near the surface in the pedosphere (Fig.
9.la), where biological activity accelerates the alteration of rocks and minerals. Significant weathering
also occurs below the pedosphere, in the so-called
saprolite (cf. Fig. 9.3). Then the base ofthe saprolite
may represent the most active front of weathering.
The term regolith is mostly used in a broader sense; it
signifies the mantle of unconsolidated and fragmental rock material covering the bedrock whether
formed in place or transported.
In general, large, thermodynamically unstable
minerals are transformed into smaIler, stable components such as clay minerals (neo-minerals, "mineralogieal" weathering). At the same time, the. parent
minerals lose part of their solid matter by chemical
reactions and dissolution ("chemieal" weathering).
Thus, the entire alteration process should be referred
to as "biochemical-chemical-mineralogical" weathering, but it is frequently called chemical weathering.
Several minerals are completely dissolved, such as
carbonate minerals in humid regions.
Chemical-mineralogical and biological weathering
are dominant in wet climates, whereas in arid climates rocks are disintegrated mainly by mechanical
processes. The products of chemical weathering are
best developed in lowland areas or on flat plateaus,
where the altered material of the saprolite cannot be
removed readily from its parent rock. Hence, deeply
weathered profiles of completely altered parent rock
are common in tropical lowlands or on large
nondissected plateaus (Figs. 9.la and 9.3d). Regions
of high topographie relief in conjunction with substantial rainfall are most important in delivering particulate matter into rivers.
Erosion on hillslopes is controlled by both the
weathering rate and transport processes which remove the disintegrated, loose material at the surface.
Erosion may be "weathering-limited', if the capacity
of the transport processes exceeds the rate at which
erodible material is generated by weathering (cf. Fig.
9.3c and d). As a result, the bedrock is exposed and
the slopes tend to steepen, thus further promoting
mechanical denudation. In contrast, erosion is
"transport-limited' if more material can be supplied
by weathering than can be removed by transport proChapter 9 Denudation and Mass Transport
cesses. In this case, thick soil profiles develop, and
transport of materials in solution predominates.
The principal relationship between climate and
weathering products of the source area with, e.g.,
granitic bedrock, providing both clastic material and
dissolved river load for a sedimentary basin, is summarized in Fig. 9.lc. Tropical regions tend to deliver
predominantly neo-minerals and a small portion of
the most stable minerals from pre-existing bedrocks
as terrigenous sediment components. Temperate, humid regions supply both neo-minerals and stable preexisting minerals, whereas arid regions deli ver comparatively unstable minerals. This simplified model
may be modified by grain sorting during sediment
transport. Fine-grained suspended river load moves
much faster and usually over longer distances than
the coarser, sandy and gravelly bed load (Fig. 9.1b).
It is therefore usually separated from the bed load.
The dissolved river loads in the three climatic
zones also differ markedly both in the concentrations
of various chemical species and in their freight rates
(Fig. 9.lc). Due to intense precipitation and chemical
weathering, more solid matter per unit area and time
can be dissolved in the wet tropics than in other
zones, provided the bedrock is not protected by thick,
clay-rich vegetated soil covers. In arid regions, mechanical processes are more important than dissolution. These and some further aspects will be further
discussed quantitatively in Sections 9.2 and 9.3.
9.1.2 Modern Soils and Paleosols (Overview)
Introduction
Soils play an important part not only in providing
various erodible materials for sediment transport and
basin filling, but also as indicators of paleoclimate
and depositional or erosional processes which have
been operating in the past. Paleosols have also become a significant tool in continental sequence stratigraphy (cf. Chap. 7.6). However, the genesis of
modem soils (pedogenesis) and older buried soils
(i.e. paleosols) and the degree of their preservation,
partial destruction, or diagenetic overprint are complex processes. The following brief overview mainly
stresses sedimentological aspects.
Soil forming processes, soil classification, and paleosols
are treated in several special publications and books (e.g.
Wright 1986; Reinhardt and Sigleo 1988; Retallack 1990;
Martini and Chesworth 1992). Duricrusts are described e.g.
by Wopfner (1978), Twidale and Milnes (1983), Milnes
and Thiry (1992).
Soils develop at the land surface under different climates, hydrological regimes, vegetation and other
biological activities. They are further influenced by
the nature of the parent rocks or unconsolidated sedi-