492
Lake Bonneville covered a much larger area (51 300 km 2;
drainage area 95 000 km 2 ) in the pluvial stages ofthe Pleistocene than the present Great Salt Lake (4360 km 2, drainage area 21 000 km 2 , Fig. 11.6b). During its last highstand
at about 15 ka B.P., the lake level stood -300 m higher and
the lake area was apptoximately 12 times larger than at
present (e.g. Benson et al. 1990). Somewhat later, the outlet to the Snake River cut a 100 m deep valley into young
sediments at the northern lake margin and caused an extreme flood (Jarrett and Malde 1987). Then the lake level
fell below this datum leading to a closed lake system.
When the lake level had fallen below the sills now separating the Great Salt Lake basin from the Utah Lake and
Sevier Lake basins, the drainage area of the Great Salt Lake
was reduced to its present size. Aseries ofboreholes (up to
about 300 m in depth) and radiometrie dating of trephra
layers have revealed that the Lake Bonneville basin. persisted at least for about 3.3 Ma (Williarns 1994). The sedimentation rates in central bore locations were 120 to 230
mrnIka.
The mean denudation rate is estimated in the following
way. The sediment accumulated at a mean rate of 120
mmlka during the past 0.8 Ma (the time period documented
best in 5 cores). Half of the lake area (25 000 km 2) during
its highstand was covered with sediment deposited at this
rate. Then the mean denudation rate in the drainage area of
-70 000 km 2 (the total Lake Bonneville drainage area minus the area of sediment accumulation) is 43 mmlka, corresponding to a specific sediment yield of about 110 tlkm 2/a
(mean rock density 2.5 glcm 3 ). With AjA b =4.9 ofthe present Great Salt Lake basin and the long-term denudation
rate of the entire dr!linage area, the mean clastic sedimentation rate in the Great Salt lake should be 210 mrnIka. The
measured mean sedimentation rate (max. value) in the lake
basin during the Quaternary was about 170 mrnIka (Oviatt
et al. 1994).
In the larger area of the Great Basin of N orth America,
the mean denudation rate was about 60 mmlka during the
past 100 to 1000 ka (Leeder 1997). It varied ftom lower
values during the humid periods to higher values during
the semiarid intervals such as in the Holocene.
The Caspian Sea, Aral Sea, and Chad Basin
At present, the Chad basin in N Africa, the Caspian
Sea, and the Aral Sea in Eurasia represent closed
basins. They occupy the centers of large topographic
depressions with desert climates and dune fields.
Therefore eolian dust plays some part in the sediment
fills of these basins (Khrustalyov and Artiukhin
1992). Lake Chad lies above sea level, the water
level of the Capsian Sea is about -25 m and that of
the Aral Sea +50 m. In wetter periods during the
Pleistocene, the Chad Lake was larger than today and
had an outflow to the Niger River; the Aral Sea
reached an elevation allowing overflow to the Caspian Sea and the Caspian Sea in turn had an outlet to
the Black Sea (Letolle and Mainguet 1996). The
present-day water budget of the two Eurasian basins
is strongly disturbed by artificial dams and irrigation
measures along the rivers feeding the basins.
Chapter 11 Sediment Supply, Subsidence, and Basin Fill
Chad basin. This basin in the central part of North
Africa, described in detail by Gac (1980), is a completely closed basin with an internal drainage system
(Fig. 11.7a).
The total drainage area of the lake comprises an area of
approximately 2 x 10 6 km 2 , while the area ofthe lake itself
varies in response to dry and wet years around an average
area of 2.1 x 10 4 km 2 (hence Ad/ Ab=95). Evaporation from
the lake is 2150 mmla. About 6 ka ago, when the climate in
North Africa was more humid, the lake was open with an
outflow to the southwest. Today, the major part of the
drainage area is hot and arid (Sahara Desert). Most of the
dissolved and detrital river load comes from subtropical
highlands to the southeast comprising about 30% of the
drainage area. Their sediment yield is so high that an average of 10.5 tlkm 2l a for the total drainage area is reached.
Mean chemical denudation amounts to 3.6 tIkm 2/a. These
low rates result from both low relief and dry climate in
large parts of the drainage area as weil as from thick, extensive soil cover on top of crystalline parent rocks.
If both mechanical and chemical river loads were
evenly spread over the present-day lake area, asolid
sediment 1ayer of about 550 rnmJka would result. The
actual thickness of wet Ho10cene sediments (4-6 m in
10 ka), consisting of terrigenous silty clay, carbonate
and biogenic silica, yields an average (uncompacted)
sedimentation rate of about 500 mmlka (compacted
200 mmIka). The modem mechanical and chemical
denudation rates, derived from the river loads, are
DR",e=4 mmlka and DR ch =1.5 rnmJka. The longerterm mean mechanical denudation rate calculated
from the Holocene sediment volume of the lake is 2
mm!ka. The observed lower sedimentation rate in the
lake rnay reflect the larger extent ofthe lake 6 ka aga
and sediment storage along the low-gradient parts of
the river courses.
Aral Sea. This present-day lake basin (Fig. 11. 7b)
rests on old continental basement. Facies and limited
thickness (-2 km) of Mesozoic and younger sediments indicate long-term slow subsidence and sediment accumulation in the order of 20 rnmJka. Quaternary fluvial and deltaic deposits of the lake reach
thicknesses of 20-140 m (Letolle and Mainguet
1996), corresponding to mean sedimentation rates of
at least 10-70 mmlka.
The modern sedimentation rate is much higher (about 1500
to 2000 mmlka for a sediment density of 1.5 glcm 3). This
value comes from a rnass balance of the river and air-borne
loads (Khrustalyo'v and Artiukhin 1992) and matches dated
sediments in the lake center, containing some carbonates
and evaporites (mirabilite).
The high modern sedimentation rate demonstrates a situation in which the lake basin is overfed with sediment. This
may result from the continued reduction in lake area with
time. Because the present-day rapid sediment accumulation
cannot be compensated for by subsidence, the entire water
body in the closed basin will be slowly raised in the future
Lake Bonneville covered a much larger area (51 300 km 2;
drainage area 95 000 km 2 ) in the pluvial stages ofthe Pleistocene than the present Great Salt Lake (4360 km 2, drainage area 21 000 km 2 , Fig. 11.6b). During its last highstand
at about 15 ka B.P., the lake level stood -300 m higher and
the lake area was apptoximately 12 times larger than at
present (e.g. Benson et al. 1990). Somewhat later, the outlet to the Snake River cut a 100 m deep valley into young
sediments at the northern lake margin and caused an extreme flood (Jarrett and Malde 1987). Then the lake level
fell below this datum leading to a closed lake system.
When the lake level had fallen below the sills now separating the Great Salt Lake basin from the Utah Lake and
Sevier Lake basins, the drainage area of the Great Salt Lake
was reduced to its present size. Aseries ofboreholes (up to
about 300 m in depth) and radiometrie dating of trephra
layers have revealed that the Lake Bonneville basin. persisted at least for about 3.3 Ma (Williarns 1994). The sedimentation rates in central bore locations were 120 to 230
mrnIka.
The mean denudation rate is estimated in the following
way. The sediment accumulated at a mean rate of 120
mmlka during the past 0.8 Ma (the time period documented
best in 5 cores). Half of the lake area (25 000 km 2) during
its highstand was covered with sediment deposited at this
rate. Then the mean denudation rate in the drainage area of
-70 000 km 2 (the total Lake Bonneville drainage area minus the area of sediment accumulation) is 43 mmlka, corresponding to a specific sediment yield of about 110 tlkm 2/a
(mean rock density 2.5 glcm 3 ). With AjA b =4.9 ofthe present Great Salt Lake basin and the long-term denudation
rate of the entire dr!linage area, the mean clastic sedimentation rate in the Great Salt lake should be 210 mrnIka. The
measured mean sedimentation rate (max. value) in the lake
basin during the Quaternary was about 170 mrnIka (Oviatt
et al. 1994).
In the larger area of the Great Basin of N orth America,
the mean denudation rate was about 60 mmlka during the
past 100 to 1000 ka (Leeder 1997). It varied ftom lower
values during the humid periods to higher values during
the semiarid intervals such as in the Holocene.
The Caspian Sea, Aral Sea, and Chad Basin
At present, the Chad basin in N Africa, the Caspian
Sea, and the Aral Sea in Eurasia represent closed
basins. They occupy the centers of large topographic
depressions with desert climates and dune fields.
Therefore eolian dust plays some part in the sediment
fills of these basins (Khrustalyov and Artiukhin
1992). Lake Chad lies above sea level, the water
level of the Capsian Sea is about -25 m and that of
the Aral Sea +50 m. In wetter periods during the
Pleistocene, the Chad Lake was larger than today and
had an outflow to the Niger River; the Aral Sea
reached an elevation allowing overflow to the Caspian Sea and the Caspian Sea in turn had an outlet to
the Black Sea (Letolle and Mainguet 1996). The
present-day water budget of the two Eurasian basins
is strongly disturbed by artificial dams and irrigation
measures along the rivers feeding the basins.
Chapter 11 Sediment Supply, Subsidence, and Basin Fill
Chad basin. This basin in the central part of North
Africa, described in detail by Gac (1980), is a completely closed basin with an internal drainage system
(Fig. 11.7a).
The total drainage area of the lake comprises an area of
approximately 2 x 10 6 km 2 , while the area ofthe lake itself
varies in response to dry and wet years around an average
area of 2.1 x 10 4 km 2 (hence Ad/ Ab=95). Evaporation from
the lake is 2150 mmla. About 6 ka ago, when the climate in
North Africa was more humid, the lake was open with an
outflow to the southwest. Today, the major part of the
drainage area is hot and arid (Sahara Desert). Most of the
dissolved and detrital river load comes from subtropical
highlands to the southeast comprising about 30% of the
drainage area. Their sediment yield is so high that an average of 10.5 tlkm 2l a for the total drainage area is reached.
Mean chemical denudation amounts to 3.6 tIkm 2/a. These
low rates result from both low relief and dry climate in
large parts of the drainage area as weil as from thick, extensive soil cover on top of crystalline parent rocks.
If both mechanical and chemical river loads were
evenly spread over the present-day lake area, asolid
sediment 1ayer of about 550 rnmJka would result. The
actual thickness of wet Ho10cene sediments (4-6 m in
10 ka), consisting of terrigenous silty clay, carbonate
and biogenic silica, yields an average (uncompacted)
sedimentation rate of about 500 mmlka (compacted
200 mmIka). The modem mechanical and chemical
denudation rates, derived from the river loads, are
DR",e=4 mmlka and DR ch =1.5 rnmJka. The longerterm mean mechanical denudation rate calculated
from the Holocene sediment volume of the lake is 2
mm!ka. The observed lower sedimentation rate in the
lake rnay reflect the larger extent ofthe lake 6 ka aga
and sediment storage along the low-gradient parts of
the river courses.
Aral Sea. This present-day lake basin (Fig. 11. 7b)
rests on old continental basement. Facies and limited
thickness (-2 km) of Mesozoic and younger sediments indicate long-term slow subsidence and sediment accumulation in the order of 20 rnmJka. Quaternary fluvial and deltaic deposits of the lake reach
thicknesses of 20-140 m (Letolle and Mainguet
1996), corresponding to mean sedimentation rates of
at least 10-70 mmlka.
The modern sedimentation rate is much higher (about 1500
to 2000 mmlka for a sediment density of 1.5 glcm 3). This
value comes from a rnass balance of the river and air-borne
loads (Khrustalyo'v and Artiukhin 1992) and matches dated
sediments in the lake center, containing some carbonates
and evaporites (mirabilite).
The high modern sedimentation rate demonstrates a situation in which the lake basin is overfed with sediment. This
may result from the continued reduction in lake area with
time. Because the present-day rapid sediment accumulation
cannot be compensated for by subsidence, the entire water
body in the closed basin will be slowly raised in the future