420
also to climatically unstable conditions. The presence
of thick paleosols indicates long time periods of stable land surfaces.
Cumulate, Composite, and Compound Paleosols
In addition to irregular sediment aggradation and erosion, climate change may generate different types of
soil on top of each other. The interrelationship of
these factors controls primary pedogenesis, overprinting of older by younger soils as well as the preservation or destruction of paleosols in sedimentary
sequences. Some of the numerous possible interactions are shown in Fig. 9.4. They may be subdivided
into several groups (Mariott and Wright 1993):
(a) Non-deposition, neither erosion nor climate
change. The soil matures and may become
thicker with time.
(b) Climate change leads to overprint of the initial
soil profile (polygenetic profile). If the new climatic conditions persist for a long time the record of older soil profile(s) may be completely
obliterated.
(c) Erosion (truncation) causes the soil to penetrate
deeper into the parent rock (reforrned profile).
(d) Continued slow (regular) deposition allows the
soil to thicken with time (cumulate soil).
(e) Episodic rapid deposition creates interruptions
and repetitions of the soil profile (composite soil
and compound soil sets). Prior to deposition the
original soil profile can be truncated (compound
truncated profile).
These principal types of paleosol profile may be
modified by further complications caused by changes
in the characteristics of the parent rock, differences
in relief, and types of the accumulating sediment
which may contain redeposited soil material (e.g. in
floodplains), etc. Specific soils also develop on top
of dry lake sediments, on carbonate rocks subjected
to karstification, or on quartzitic sandstones. Finally,
it should be mentioned that pedogenesis in the Precambrian and early Paleozoic periods was controlled
by processes partly different from those in the modem world because plant life comparable to the present situation did not yet exist on land (cf. Sect. 6.5).
9.1.3 Weathering and Composition of
Suspended River Loads (Overview)
General Aspects and Problems
With respect to the transport capacity of running water and wind, the weathering products of various rock
types can be subdivided into two groups: (1) relaChapter 9 Denudation and Mass Transport
tively coarse- (gravel and sand) and fine-grained material (clayand silt-size fractions). If the provenance
of sediments is of primary interest, studies on the
coarse fraction (including heavy minerals) renders
good results, whereas the fine fraction is more difficult to evaluate. In terms of quantity, however, the
fine fraction is more important.
As will be demonstrated in Section 9.3, the solid
river load mainly consists of fine-grained particles
carried in suspension during high runoff and floods
(e.g., Pye 1994). For this reason it is not surprising
that clays and claystones (argillites), clayey siltstones, muds tones, marlstones, shales and slates (including schists) are the most abundant group of all
sedimentary rocks. In addition, marine hemipelagic
and pelagic sediments contain high proportions of
allochthonous terrigenous material apart from their
biogenic components.
The suspended particle flux of rivers consists essentially of a limited number of minerals which can
be ascribed to two sources:
- Primary detrital minerals inherited from the parent rocks.
- Secondary minerals (neo-minerals) generated by
chemical-mineralogical weathering in the drainage
area.
An additional group of fine-grained minerals incIudes
authigenetic and diagenetic clay minerals, glauconite and
pyrite, which form in the depositional environment or
within the sediment. These minerals are not further discussed in this section. The impact of deep-burial diagenesis
on the clay mineral suite of fine-grained sedimentary rocks
is mentioned in Sect. 13.3.
Provenance studies based on the mineralogical composition of the sand-size fraction have to be treated
with caution. Some of these studies may reflect the
composition of the weathering profile mantling
source rocks rather than bedrock (Nesbitt et al.
1996), particularly in regions where substantial
chemical weathering has occurred prior to erosion.
Analysing the mineralogical composition of suspended river loads has its problems. Data gained by
different authors and methods on the same river system sometimes display considerable scatter. A common explanation of these discrepancies is the investigation of sampies of differing grain size distribution.
Sampies taken during peak floods contain higher proportions of silt than those collected during lower runoff. If only the ::;2 J.1m fraction is analyzed, clay minerals would predominate, whereas quartz and other
source rock minerals would be rare. The presence of
X-rayamorphous matter can add to the uncertainties
regarding quantification of the mineralogical composition of fine-grained material.
also to climatically unstable conditions. The presence
of thick paleosols indicates long time periods of stable land surfaces.
Cumulate, Composite, and Compound Paleosols
In addition to irregular sediment aggradation and erosion, climate change may generate different types of
soil on top of each other. The interrelationship of
these factors controls primary pedogenesis, overprinting of older by younger soils as well as the preservation or destruction of paleosols in sedimentary
sequences. Some of the numerous possible interactions are shown in Fig. 9.4. They may be subdivided
into several groups (Mariott and Wright 1993):
(a) Non-deposition, neither erosion nor climate
change. The soil matures and may become
thicker with time.
(b) Climate change leads to overprint of the initial
soil profile (polygenetic profile). If the new climatic conditions persist for a long time the record of older soil profile(s) may be completely
obliterated.
(c) Erosion (truncation) causes the soil to penetrate
deeper into the parent rock (reforrned profile).
(d) Continued slow (regular) deposition allows the
soil to thicken with time (cumulate soil).
(e) Episodic rapid deposition creates interruptions
and repetitions of the soil profile (composite soil
and compound soil sets). Prior to deposition the
original soil profile can be truncated (compound
truncated profile).
These principal types of paleosol profile may be
modified by further complications caused by changes
in the characteristics of the parent rock, differences
in relief, and types of the accumulating sediment
which may contain redeposited soil material (e.g. in
floodplains), etc. Specific soils also develop on top
of dry lake sediments, on carbonate rocks subjected
to karstification, or on quartzitic sandstones. Finally,
it should be mentioned that pedogenesis in the Precambrian and early Paleozoic periods was controlled
by processes partly different from those in the modem world because plant life comparable to the present situation did not yet exist on land (cf. Sect. 6.5).
9.1.3 Weathering and Composition of
Suspended River Loads (Overview)
General Aspects and Problems
With respect to the transport capacity of running water and wind, the weathering products of various rock
types can be subdivided into two groups: (1) relaChapter 9 Denudation and Mass Transport
tively coarse- (gravel and sand) and fine-grained material (clayand silt-size fractions). If the provenance
of sediments is of primary interest, studies on the
coarse fraction (including heavy minerals) renders
good results, whereas the fine fraction is more difficult to evaluate. In terms of quantity, however, the
fine fraction is more important.
As will be demonstrated in Section 9.3, the solid
river load mainly consists of fine-grained particles
carried in suspension during high runoff and floods
(e.g., Pye 1994). For this reason it is not surprising
that clays and claystones (argillites), clayey siltstones, muds tones, marlstones, shales and slates (including schists) are the most abundant group of all
sedimentary rocks. In addition, marine hemipelagic
and pelagic sediments contain high proportions of
allochthonous terrigenous material apart from their
biogenic components.
The suspended particle flux of rivers consists essentially of a limited number of minerals which can
be ascribed to two sources:
- Primary detrital minerals inherited from the parent rocks.
- Secondary minerals (neo-minerals) generated by
chemical-mineralogical weathering in the drainage
area.
An additional group of fine-grained minerals incIudes
authigenetic and diagenetic clay minerals, glauconite and
pyrite, which form in the depositional environment or
within the sediment. These minerals are not further discussed in this section. The impact of deep-burial diagenesis
on the clay mineral suite of fine-grained sedimentary rocks
is mentioned in Sect. 13.3.
Provenance studies based on the mineralogical composition of the sand-size fraction have to be treated
with caution. Some of these studies may reflect the
composition of the weathering profile mantling
source rocks rather than bedrock (Nesbitt et al.
1996), particularly in regions where substantial
chemical weathering has occurred prior to erosion.
Analysing the mineralogical composition of suspended river loads has its problems. Data gained by
different authors and methods on the same river system sometimes display considerable scatter. A common explanation of these discrepancies is the investigation of sampies of differing grain size distribution.
Sampies taken during peak floods contain higher proportions of silt than those collected during lower runoff. If only the ::;2 J.1m fraction is analyzed, clay minerals would predominate, whereas quartz and other
source rock minerals would be rare. The presence of
X-rayamorphous matter can add to the uncertainties
regarding quantification of the mineralogical composition of fine-grained material.