446
The Late Glacial-Holocene Transition
New data for the Pleistocene and Holocene mechanical denudation of the entire Alps, derived from the
fillings of perialpine lakes, demonstrate that the average denudation rate since the last glacial maximum
(~17 ka B.P.) was about 5 times higher than the Holocene rate (620 vs. 125 mmlka, Hinderer 1999).
During the total Quaternary, the average denudation
rate in the Alps (--400 mmlka) was two to three times
higher than in the late Oligocene and Miocene
(Kuhlemann et al. 1999).
Data for the fjords ofwestern Norway which were
shaped by the advancing ice during the glacial periods but later filled with sediment displaya reduction
in denudation similar to that found for the Alps.
Late-glacial (since the last glacial maximum) denudation in the catchment of Sognefjorden (520 mmlka)
was 7.4 times higher than denudation in the Holocene (Bogen 1996; Aarseth 1997).
The sediments of the Black Sea also document the
influence of climatic change (Fig. 9.19, Degens et al.
1991, cf. Sect. 11.2.3). The present-day mean mechanical denudation rate in its drainage area (~30
mmIka), as derived from river loads, is similar to the
rate (~40 mmlka) calculated from the carbonate-free
solid sediment for the past 5 ka. During the last
deglaciation (15 to 8 ka B.P.) the sediment yield of
the rivers was two to four times greater than at present.
9.5.3 Regions ofVery Siow Denudation
Some of the great Precambrian shields which were
not affected by the Pleistocene glaciations have
shown little evidence of denudation for very long
time periods (e.g. Fairbridge and Fink! (1980). These
tectonically stahle areas, such as parts of the former
Gondwana continent, were leveled many hundred
million years ago and since then have kept an elevation most of the time slightly above base level of erosion. Intermittently, part of their almost planar erosion surface (peneplain) has been buried by a thin
veneer of sediment and exhumed by subsequent erosion.
This is documented by the presence of relic sediments of
different ages at the edges of these cratonic regions. In Africa and particularly in western Australia, such multiple
"stacked" veneers are superimposed and subparallel not
only to each other, but also to the original cratonic surface.
They are often separated from the basement and from one
another by paleosols and duricrusts (e.g., silcrete or
lateritic ernsts). In more central parts of the peneplain,
which remained uncovered or were rarely buried by sediments, very deep, old weathering profiles may be preserved
(see also, e.g., Firman 1988). The total denudation rate in
the humid tropical environment of the Guayana Shield
Chapter 9 Denudation and Mass Transport
(Venezuela, Colombia, and Brazil) was found to be only
about 10 mmlka, with equal contribution from chemie al
(mostly dissolved silica) and mechanical erosion (Edmond
et al. 1995). Here, a thick lateritic regolith with kaolinite
and gibbsite has developed on basement rocks.
The long-term net surface lowering of these regions
can be extremely slow (in Northwest Australia 0.1 to
0.2 mlMa, in Southeast Australia 3 to 8 mlMa, in the
Chad Basin 0.5 to 2 mlMa; Table 9.1; cf. Sect.
11.2.3). Some isostatic rebound enables denudation
to continue for long time periods, even when thermotectonically generated uplift is absent.
9.5.4 Modern vs. Long-Term Denudation Rates
In the context of this volume geological time periods
of some 100 ka to many miIlions of years as weIl as
mean denudation rates of large areas (> 1 00 000 km 2 )
are of interest. Therefore it is important to know
whether or not modern sediment yields of large
drainage systems can be used in studies of ancient
denudation/sediment-accumulation systems (cf.
Sects. 11.2 and 11.3). Drastic increases in man-induced soil erosion have been reported from many
countries and seem to restrict the usefulness of modern sediment yields in basin analysis. However, significant soil erosion normally takes place in areas of
limited size, specific natural characteristics, and intensive land use, whereas the average sediment yields
of many large drainage systems often are less affected by human activities. This is demonstrated for
two examples of mountainous regions and two larger
areas ofmixed relief and land use (Fig. 9.16), where
modern and long-term values for sediment yield or
mechanical denudation have been determined independently from each other.
- Tbe large Tarbela reservoir at the Indus River near
Islarnabad in northern Pakistan has a drainage area (171
000 km 2 ) comprising ab out one third of that of the total
Himalayas. A great part of the storage capacity of the dam
site was filled with sediment after 20 years of operation.
Tbe mean specific sediment yield ofthe upper Indus river,
as calculated from the sediment storage in the reservoir, is
1170 t/km 2 /a, corresponding to a mean mechanical denudation rate of 425 m1Ma in the northwestern Himalaya (mean
rock density 2.75 glcm 3 ). In this case, the modern denudation rate can be compared with the long-term (past 20 Ma)
denudation in the entire Himalayas (360 mlMa, cf. Sect.
11.5.5; Einseie and Hinderer 1997). In the High Himalayan
crystalline chain, the rate was higher (about 1000 m1Ma).
Only about 5% of these rates are caused by chemical denudation.
- Tbe relatively large Sefid-Rud reservoir in northwestern
Iran drains an area of 56 000 km 2 of semiarid c1imate (mean
precipitation 350 mm1a) and high relief in the Elburs
mountains. Tbe average sediment yield of the river basin,
partly containing unconsolidated c1astic rocks and volcanic
tuff, was ca. 900 t/km 2 /a (or 360 mmlka) for the first 17
The Late Glacial-Holocene Transition
New data for the Pleistocene and Holocene mechanical denudation of the entire Alps, derived from the
fillings of perialpine lakes, demonstrate that the average denudation rate since the last glacial maximum
(~17 ka B.P.) was about 5 times higher than the Holocene rate (620 vs. 125 mmlka, Hinderer 1999).
During the total Quaternary, the average denudation
rate in the Alps (--400 mmlka) was two to three times
higher than in the late Oligocene and Miocene
(Kuhlemann et al. 1999).
Data for the fjords ofwestern Norway which were
shaped by the advancing ice during the glacial periods but later filled with sediment displaya reduction
in denudation similar to that found for the Alps.
Late-glacial (since the last glacial maximum) denudation in the catchment of Sognefjorden (520 mmlka)
was 7.4 times higher than denudation in the Holocene (Bogen 1996; Aarseth 1997).
The sediments of the Black Sea also document the
influence of climatic change (Fig. 9.19, Degens et al.
1991, cf. Sect. 11.2.3). The present-day mean mechanical denudation rate in its drainage area (~30
mmIka), as derived from river loads, is similar to the
rate (~40 mmlka) calculated from the carbonate-free
solid sediment for the past 5 ka. During the last
deglaciation (15 to 8 ka B.P.) the sediment yield of
the rivers was two to four times greater than at present.
9.5.3 Regions ofVery Siow Denudation
Some of the great Precambrian shields which were
not affected by the Pleistocene glaciations have
shown little evidence of denudation for very long
time periods (e.g. Fairbridge and Fink! (1980). These
tectonically stahle areas, such as parts of the former
Gondwana continent, were leveled many hundred
million years ago and since then have kept an elevation most of the time slightly above base level of erosion. Intermittently, part of their almost planar erosion surface (peneplain) has been buried by a thin
veneer of sediment and exhumed by subsequent erosion.
This is documented by the presence of relic sediments of
different ages at the edges of these cratonic regions. In Africa and particularly in western Australia, such multiple
"stacked" veneers are superimposed and subparallel not
only to each other, but also to the original cratonic surface.
They are often separated from the basement and from one
another by paleosols and duricrusts (e.g., silcrete or
lateritic ernsts). In more central parts of the peneplain,
which remained uncovered or were rarely buried by sediments, very deep, old weathering profiles may be preserved
(see also, e.g., Firman 1988). The total denudation rate in
the humid tropical environment of the Guayana Shield
Chapter 9 Denudation and Mass Transport
(Venezuela, Colombia, and Brazil) was found to be only
about 10 mmlka, with equal contribution from chemie al
(mostly dissolved silica) and mechanical erosion (Edmond
et al. 1995). Here, a thick lateritic regolith with kaolinite
and gibbsite has developed on basement rocks.
The long-term net surface lowering of these regions
can be extremely slow (in Northwest Australia 0.1 to
0.2 mlMa, in Southeast Australia 3 to 8 mlMa, in the
Chad Basin 0.5 to 2 mlMa; Table 9.1; cf. Sect.
11.2.3). Some isostatic rebound enables denudation
to continue for long time periods, even when thermotectonically generated uplift is absent.
9.5.4 Modern vs. Long-Term Denudation Rates
In the context of this volume geological time periods
of some 100 ka to many miIlions of years as weIl as
mean denudation rates of large areas (> 1 00 000 km 2 )
are of interest. Therefore it is important to know
whether or not modern sediment yields of large
drainage systems can be used in studies of ancient
denudation/sediment-accumulation systems (cf.
Sects. 11.2 and 11.3). Drastic increases in man-induced soil erosion have been reported from many
countries and seem to restrict the usefulness of modern sediment yields in basin analysis. However, significant soil erosion normally takes place in areas of
limited size, specific natural characteristics, and intensive land use, whereas the average sediment yields
of many large drainage systems often are less affected by human activities. This is demonstrated for
two examples of mountainous regions and two larger
areas ofmixed relief and land use (Fig. 9.16), where
modern and long-term values for sediment yield or
mechanical denudation have been determined independently from each other.
- Tbe large Tarbela reservoir at the Indus River near
Islarnabad in northern Pakistan has a drainage area (171
000 km 2 ) comprising ab out one third of that of the total
Himalayas. A great part of the storage capacity of the dam
site was filled with sediment after 20 years of operation.
Tbe mean specific sediment yield ofthe upper Indus river,
as calculated from the sediment storage in the reservoir, is
1170 t/km 2 /a, corresponding to a mean mechanical denudation rate of 425 m1Ma in the northwestern Himalaya (mean
rock density 2.75 glcm 3 ). In this case, the modern denudation rate can be compared with the long-term (past 20 Ma)
denudation in the entire Himalayas (360 mlMa, cf. Sect.
11.5.5; Einseie and Hinderer 1997). In the High Himalayan
crystalline chain, the rate was higher (about 1000 m1Ma).
Only about 5% of these rates are caused by chemical denudation.
- Tbe relatively large Sefid-Rud reservoir in northwestern
Iran drains an area of 56 000 km 2 of semiarid c1imate (mean
precipitation 350 mm1a) and high relief in the Elburs
mountains. Tbe average sediment yield of the river basin,
partly containing unconsolidated c1astic rocks and volcanic
tuff, was ca. 900 t/km 2 /a (or 360 mmlka) for the first 17