11.3 Dynamic Denudation Accumulation Systems
active subsidence, basin geometry, climatic conditions, water chemistry, and types and rates of sedimentation (e.g. Reading 1986; Cohen 1989). Therefore, open as weH as closed lake basins of different
water depths developed. The levels of these lakes
fluctuated by up to several hundred meters during the
past 15 ka and earlier due to climatic change, accompanied by drastic changes in sediment influx (e.g.
increased "lowstand shedding", cf. Sect. 7.6). In the
context of this chapter, a comparison of Lake
Tanganyika and Lake Turkana may be of interest (cf.
Fig. 12.4; for more details see Einseie and Hinderer
1997).
Evaluating the rates of subsidence, sediment influx
and mean sedimentation reveals that Lake
Tanganyika is under-supplied with sediment and
deepening while Lake Turkana is over-supplied and
shaHowing.
Lake Tanganyika is at present an open lake with little
outflow. The sedimentation rate varies in the different
subbasins ofthis lake, but is generally in the order of ~200
mmlka for wet material (porosity n:::::: 50%). Thus, sedimentation cannot keep pace with subsidence of the basin
floor (~280 mmlka during the past 20 Ma) because sediment compaction reduces the initial sediment porosity. The
modem lake is therefore undersupplied and deepening
rather than shallowing (present mean water depth 570 m).
The present-day mean mechanical denudation rate as derived from SRme of sediment cores (AJAb=7.1) is about 4
mmlka (10 tlkm 2 /a). It only allows a mean clastic sedimentation rate of -60 mmlka (n:::::: 50%).
Lake Turkana (closed system, mean water depth 31 m).
Thebasin floor is subsiding at a rate of about 130 mmlka,
but the modem mean sedimentation rate (n:::::: 50%) is
-1200 mmlka and indicates that the basin is presently oversupplied and being filled up rapidly with sediment. The
modem (past 2 ka) mean mechanical denudation rate in the
drainage area (AJAb=9.7) is 93 mmlka or 230 tlkm 2 /a.
The contrasting "state of basin filling" of these two
lakes mainly results from the great difference in the
denudation rates of their drainage areas. The Ai Ab
ratios of the two lakes are almost the same and their
subsidence rates are of the same order of magnitude.
In the catchment of Lake Turkana D~e is higher by
a factor of >20 than in the Tanganyika catchment.
This reflects the presence of young, partially nonindurated volcanic rocks and volcaniclastics around
Lake Turkana compared with older crystalline rocks
in the catchment of Lake Tanganyika as well as differences in climate (serni-arid to subtropical versus
subtropical to tropical).
497
11.3.3 Extensional Basin Models
Rift Basins with Constant Sediment Supply
In the context of this chapter three principal types of
basins are of interest: (1) basins of more or less constant areal extent (Sect. 11.3.1), (2) extensional basins, and (3) closing basins. The plate tectonic setting
and general evolution of sedimentary basins is discussed in Chaps. 1 and 12.
The most important representatives of extensional
basins are rift basins, growing ocean basins, and pullapart basins (Fig. 11.11a). For all of them their size
increases with time leading to the general trend of
decreasing rates of terrigenous sedimentation. The
simplest case is when the rate of sediment input is
kept constant. Such a situation is demonstrated quantitatively for rift basins (Fig. 11.12) with the following assumptions (Schlische and Olsen 1990; for further details see Schlische 1992):
- The basin represents a continental full-graben or
half-graben and is bounded by planar faults dipping
at equal angles (Fig. 11.12a).
- Uniform extension causes uniform subsidence
along the boundary faults and hence the depth of the
basin increases linearly.
- The outlet of the basin is held at a constant level
with respect to an external datum line.
- The volume of fluvial sediment added to the basin
per unit time is constant; the sediment is distributed
uniforrniy over the entire basin.
This model basin may display the following stages of
evolution (Fig. 11.12a):
Stage 1. Sediment supply is large enough to keep the
narrow basin always filled with fluvial material; excess sediment and water leave the basin which is
therefore both hydrologically and sedimentologically
open. During this phase, the sedimentation rate is
constant and equal to the subsidence rate.
Stage 2. As the basin continues to grow, a point is
reached at which the sediment input can no longer
completely fill the space made available through subsidence. Consequently, the basin becomes sedimentologically closed and is transformed into a lake
or shallow sea. The sedimentation rate slows due to
the increasing size of the basin. Y ounger strata progressively onlap the basement rocks of the hanging
wall block.
Stage 3. Further development is controlled by climatic factors and the size of the hydrologic drainage
area of the basin. If water supply into the basin remains high enough to keep the basin water-filled up
to the fixed outlet, the basin deepens and the sedimentation rate continuously decreases.
active subsidence, basin geometry, climatic conditions, water chemistry, and types and rates of sedimentation (e.g. Reading 1986; Cohen 1989). Therefore, open as weH as closed lake basins of different
water depths developed. The levels of these lakes
fluctuated by up to several hundred meters during the
past 15 ka and earlier due to climatic change, accompanied by drastic changes in sediment influx (e.g.
increased "lowstand shedding", cf. Sect. 7.6). In the
context of this chapter, a comparison of Lake
Tanganyika and Lake Turkana may be of interest (cf.
Fig. 12.4; for more details see Einseie and Hinderer
1997).
Evaluating the rates of subsidence, sediment influx
and mean sedimentation reveals that Lake
Tanganyika is under-supplied with sediment and
deepening while Lake Turkana is over-supplied and
shaHowing.
Lake Tanganyika is at present an open lake with little
outflow. The sedimentation rate varies in the different
subbasins ofthis lake, but is generally in the order of ~200
mmlka for wet material (porosity n:::::: 50%). Thus, sedimentation cannot keep pace with subsidence of the basin
floor (~280 mmlka during the past 20 Ma) because sediment compaction reduces the initial sediment porosity. The
modem lake is therefore undersupplied and deepening
rather than shallowing (present mean water depth 570 m).
The present-day mean mechanical denudation rate as derived from SRme of sediment cores (AJAb=7.1) is about 4
mmlka (10 tlkm 2 /a). It only allows a mean clastic sedimentation rate of -60 mmlka (n:::::: 50%).
Lake Turkana (closed system, mean water depth 31 m).
Thebasin floor is subsiding at a rate of about 130 mmlka,
but the modem mean sedimentation rate (n:::::: 50%) is
-1200 mmlka and indicates that the basin is presently oversupplied and being filled up rapidly with sediment. The
modem (past 2 ka) mean mechanical denudation rate in the
drainage area (AJAb=9.7) is 93 mmlka or 230 tlkm 2 /a.
The contrasting "state of basin filling" of these two
lakes mainly results from the great difference in the
denudation rates of their drainage areas. The Ai Ab
ratios of the two lakes are almost the same and their
subsidence rates are of the same order of magnitude.
In the catchment of Lake Turkana D~e is higher by
a factor of >20 than in the Tanganyika catchment.
This reflects the presence of young, partially nonindurated volcanic rocks and volcaniclastics around
Lake Turkana compared with older crystalline rocks
in the catchment of Lake Tanganyika as well as differences in climate (serni-arid to subtropical versus
subtropical to tropical).
497
11.3.3 Extensional Basin Models
Rift Basins with Constant Sediment Supply
In the context of this chapter three principal types of
basins are of interest: (1) basins of more or less constant areal extent (Sect. 11.3.1), (2) extensional basins, and (3) closing basins. The plate tectonic setting
and general evolution of sedimentary basins is discussed in Chaps. 1 and 12.
The most important representatives of extensional
basins are rift basins, growing ocean basins, and pullapart basins (Fig. 11.11a). For all of them their size
increases with time leading to the general trend of
decreasing rates of terrigenous sedimentation. The
simplest case is when the rate of sediment input is
kept constant. Such a situation is demonstrated quantitatively for rift basins (Fig. 11.12) with the following assumptions (Schlische and Olsen 1990; for further details see Schlische 1992):
- The basin represents a continental full-graben or
half-graben and is bounded by planar faults dipping
at equal angles (Fig. 11.12a).
- Uniform extension causes uniform subsidence
along the boundary faults and hence the depth of the
basin increases linearly.
- The outlet of the basin is held at a constant level
with respect to an external datum line.
- The volume of fluvial sediment added to the basin
per unit time is constant; the sediment is distributed
uniforrniy over the entire basin.
This model basin may display the following stages of
evolution (Fig. 11.12a):
Stage 1. Sediment supply is large enough to keep the
narrow basin always filled with fluvial material; excess sediment and water leave the basin which is
therefore both hydrologically and sedimentologically
open. During this phase, the sedimentation rate is
constant and equal to the subsidence rate.
Stage 2. As the basin continues to grow, a point is
reached at which the sediment input can no longer
completely fill the space made available through subsidence. Consequently, the basin becomes sedimentologically closed and is transformed into a lake
or shallow sea. The sedimentation rate slows due to
the increasing size of the basin. Y ounger strata progressively onlap the basement rocks of the hanging
wall block.
Stage 3. Further development is controlled by climatic factors and the size of the hydrologic drainage
area of the basin. If water supply into the basin remains high enough to keep the basin water-filled up
to the fixed outlet, the basin deepens and the sedimentation rate continuously decreases.