494
(cf. Fig. 11.7a). The future existence of the Aral Sea depends on its water budget which is presently substantially
affected by water subtraction for irrigation. If inflow will
again exceed evaporation from the lake, then the lake will
deepen, extend and finally overflow. Simultaneously, the
sedimentation rate will decrease.
Caspian Sea. This is the world's largest closed lake
basin (Fig. 11. 7b). Its northem part is underlain by 2
km of Mesozoie and Tertiary sediments, resting on
continental ernst. This situation points to a slow
mean subsidence rate similar to that mentioned for
the Aral Sea. The shallow water depth (10 to 15 m)
indicates that sediment influx from the large northem
drainage area, eolian dust input, and autochthonous
sediment production keep the basin more or less
filled. The potential modem sedimentation rate is
higher than necessary to maintain this steady-state
condition. Surplus of sediment derived from the
north can be transported south by wave and current
action.
The middle part of the Caspian Sea is underlain by
intermediate crnst and a sedimentary sequence of
about 10 km in thickness. The present water depth in
the basin center (about 300 m) demonstrates that sediment influx from northem and nearby sources has
not completely filled this portion of the basin.
Based on the present-day river loads or sediment yields of
the various drainage systems (F ig. 11. 7b), the total sediment influx into the entire Caspian Sea is estimated to be
-500 x 10 6 tJa (summary in Einseie and Hinderer 1997).
Transformed into wet sediment (mean porosity of 0.4, conversion factor 0.63, Fig. 11.1b) this mass would yield 315
x 10 6 m 3 /a. Neglecting subsidence, a time period of -250
000 years would be necessary to replace the water volume
of 78.2 x 10 12 m 3 of the modern Caspian Sea by wet porous
sediment. However, this is an unrealistic prediction because we are dealing with a dynamic system with basin
floor subsidence, particularly so in the southern part of the
basin.
The southern Caspian Sea (Fig. 11.7b) still undergoes convergence in relation to Eurasia (~7 mmla)
and is further characterized by a large negative free
air gravity anomaly implying continuous subsidence
(Zonenshain et al. 1990). Tectonic activity in this
backarc basin reached its peak in the Upper Pliocene
and Quatemary. This is inferred from the thick sedimentary sequences in the center of the southem
Caspain Sea (up to 6000 m Pliocene and 1200 to
1500 m Quatemary sediments) as weIl as from intense folding (Narimanov 1993). This part of the basin was and probably still is subsiding at a rate on the
order of 1000 mmlka or even more. For a short time
period, we mayaiso treat this basin as a steady-state
DA system.
The result of a crnde estimation of the overall relationship between mean denudation and sedimentation
Chapter 11 Sediment Supply, Subsidence, and Basin Fill
in the southem Caspian Sea area is plotted in Fig.
9.16. The data on terrestrial sediment influx confirm
the high sedimentation rates observed in the basin.
Modern sediment influx from the surrounding mountain
ranges enables a sedimentation rate of the same order
(-1000 nun/ka, er. Sect. 9.7) as subsidence, but the considerable water depth of the southern Caspian Sea (average
330 m, maximum about 1000 m) implies that modern sediment aeeumulation lags behind subsidence. In fact, the
basin appears to have been shallower during most of the
Oligoeene to Plioeene time period.
11.3 Dynamic Denudation-Accumulation
(DA) Systems
11.3.1 Rebound by Unloading and Sediment
Load-Driven Subsidence
Long-term denudation-accumulation (DA) systems,
persisting over time periods of millions to tens of
million years, have to be viewed as dynarnic systems,
even when active tectonic motions have ceased. Then
the processes of isostasy are still generating both
rock uplift in the source area (cf. Sect. 9.6) providing
terrestrial sediment and sediment-load driven subsidence of the basin floor (cf. Chap. 8). A simple twodimensional model of this relationship is shown in
Fig. 11.8.
It is assumed that at the begin ofthe development the sediment souree had been raised by tectonic forees to an elevation of I km above sea level and had reaehed isostatie equilibrium. The floor of a dry basin (on stretehed eontinental
ernst), twiee as large as the source area, initially lay I km
below sea level, but was further lowered by 0.4 km after
filling the basin with water (Fig. 11.8a). The areas of both
sediment souree and basin remain eonstant throughout the
time eonsidered.
After a eertain time interval, depending on the denudation rate, the land surface has been lowered by .:1H=OA km
(Fig. 11.8b). To aeeomplish this lowering, a rock column
of Dj =5 x .:1H=2 km in thiekness had to be eroded to compensate for isostatic rebound (Sects. 8.1 and 9.6, Airy isostasy). The eroded material could form a sediment layer of
hs'= I km in thickness in the basin if it had a bulk density of
p's=2.5 g/cm 3 (as assumed for the eroded rocks). In reality,
the mean density of a I km thiek sedimentary column
(mostly fine grained) is lower (about Ps=1.6 g/em 3 ) and the
sediment thiekness i, therefore greater. Autoehthonous sediment production within the basin is negleeted. As a result
of isostatic adjustment to the sediment load, the basin floor
subsides by 0.64 km (Fig. 11.8b).
Ongoing denudation (now at a decreasing rate) has to
remove another 2 km of rocks in order to further lower the
land surface by 0.4 km (Fig. 11.8c). The total thickness of
compacted sediment hs' (p's=2.5 g/cm 3 ) in the basin increases to 2 km and will amount to h s =2.85 km (mean sediment density now Ps=1.75 g/cm 3 ). Although the basin floor
subsides by another 0.64 km, a small amount of the porous
(cf. Fig. 11.7a). The future existence of the Aral Sea depends on its water budget which is presently substantially
affected by water subtraction for irrigation. If inflow will
again exceed evaporation from the lake, then the lake will
deepen, extend and finally overflow. Simultaneously, the
sedimentation rate will decrease.
Caspian Sea. This is the world's largest closed lake
basin (Fig. 11. 7b). Its northem part is underlain by 2
km of Mesozoie and Tertiary sediments, resting on
continental ernst. This situation points to a slow
mean subsidence rate similar to that mentioned for
the Aral Sea. The shallow water depth (10 to 15 m)
indicates that sediment influx from the large northem
drainage area, eolian dust input, and autochthonous
sediment production keep the basin more or less
filled. The potential modem sedimentation rate is
higher than necessary to maintain this steady-state
condition. Surplus of sediment derived from the
north can be transported south by wave and current
action.
The middle part of the Caspian Sea is underlain by
intermediate crnst and a sedimentary sequence of
about 10 km in thickness. The present water depth in
the basin center (about 300 m) demonstrates that sediment influx from northem and nearby sources has
not completely filled this portion of the basin.
Based on the present-day river loads or sediment yields of
the various drainage systems (F ig. 11. 7b), the total sediment influx into the entire Caspian Sea is estimated to be
-500 x 10 6 tJa (summary in Einseie and Hinderer 1997).
Transformed into wet sediment (mean porosity of 0.4, conversion factor 0.63, Fig. 11.1b) this mass would yield 315
x 10 6 m 3 /a. Neglecting subsidence, a time period of -250
000 years would be necessary to replace the water volume
of 78.2 x 10 12 m 3 of the modern Caspian Sea by wet porous
sediment. However, this is an unrealistic prediction because we are dealing with a dynamic system with basin
floor subsidence, particularly so in the southern part of the
basin.
The southern Caspian Sea (Fig. 11.7b) still undergoes convergence in relation to Eurasia (~7 mmla)
and is further characterized by a large negative free
air gravity anomaly implying continuous subsidence
(Zonenshain et al. 1990). Tectonic activity in this
backarc basin reached its peak in the Upper Pliocene
and Quatemary. This is inferred from the thick sedimentary sequences in the center of the southem
Caspain Sea (up to 6000 m Pliocene and 1200 to
1500 m Quatemary sediments) as weIl as from intense folding (Narimanov 1993). This part of the basin was and probably still is subsiding at a rate on the
order of 1000 mmlka or even more. For a short time
period, we mayaiso treat this basin as a steady-state
DA system.
The result of a crnde estimation of the overall relationship between mean denudation and sedimentation
Chapter 11 Sediment Supply, Subsidence, and Basin Fill
in the southem Caspian Sea area is plotted in Fig.
9.16. The data on terrestrial sediment influx confirm
the high sedimentation rates observed in the basin.
Modern sediment influx from the surrounding mountain
ranges enables a sedimentation rate of the same order
(-1000 nun/ka, er. Sect. 9.7) as subsidence, but the considerable water depth of the southern Caspian Sea (average
330 m, maximum about 1000 m) implies that modern sediment aeeumulation lags behind subsidence. In fact, the
basin appears to have been shallower during most of the
Oligoeene to Plioeene time period.
11.3 Dynamic Denudation-Accumulation
(DA) Systems
11.3.1 Rebound by Unloading and Sediment
Load-Driven Subsidence
Long-term denudation-accumulation (DA) systems,
persisting over time periods of millions to tens of
million years, have to be viewed as dynarnic systems,
even when active tectonic motions have ceased. Then
the processes of isostasy are still generating both
rock uplift in the source area (cf. Sect. 9.6) providing
terrestrial sediment and sediment-load driven subsidence of the basin floor (cf. Chap. 8). A simple twodimensional model of this relationship is shown in
Fig. 11.8.
It is assumed that at the begin ofthe development the sediment souree had been raised by tectonic forees to an elevation of I km above sea level and had reaehed isostatie equilibrium. The floor of a dry basin (on stretehed eontinental
ernst), twiee as large as the source area, initially lay I km
below sea level, but was further lowered by 0.4 km after
filling the basin with water (Fig. 11.8a). The areas of both
sediment souree and basin remain eonstant throughout the
time eonsidered.
After a eertain time interval, depending on the denudation rate, the land surface has been lowered by .:1H=OA km
(Fig. 11.8b). To aeeomplish this lowering, a rock column
of Dj =5 x .:1H=2 km in thiekness had to be eroded to compensate for isostatic rebound (Sects. 8.1 and 9.6, Airy isostasy). The eroded material could form a sediment layer of
hs'= I km in thickness in the basin if it had a bulk density of
p's=2.5 g/cm 3 (as assumed for the eroded rocks). In reality,
the mean density of a I km thiek sedimentary column
(mostly fine grained) is lower (about Ps=1.6 g/em 3 ) and the
sediment thiekness i, therefore greater. Autoehthonous sediment production within the basin is negleeted. As a result
of isostatic adjustment to the sediment load, the basin floor
subsides by 0.64 km (Fig. 11.8b).
Ongoing denudation (now at a decreasing rate) has to
remove another 2 km of rocks in order to further lower the
land surface by 0.4 km (Fig. 11.8c). The total thickness of
compacted sediment hs' (p's=2.5 g/cm 3 ) in the basin increases to 2 km and will amount to h s =2.85 km (mean sediment density now Ps=1.75 g/cm 3 ). Although the basin floor
subsides by another 0.64 km, a small amount of the porous