484
- For TE=100%, the mean sedimentation rate, S~e'
of terrigenous sediment in the basin is a function of
D~e and proportional to the ratio of the drainage
area and the basin area, Ai Ab:
(11.1 )
This simple equation is only valid if the bulk densities of both the eroded rocks and the accumulated
sediments (compacted solid material) are equal. To
find the actual sedimentation rate of porous sediment
the conversion factors listed in Fig. 11.1 b have to be
used.
- Vice versa, the mean mechanical denudation rate in
the source area is
(l1.2a)
D~e may be used as a proxy for the reconstruction
of the topography (relief) and climate of a paleodrainage area.
This concept can be used for systems where the sizes of
both the source area and the basin area are known, as for
modem and geologically young denudation-accumulation
systems. In some cases, the source area of sediment (e.g. a
mountain range) is not identical with the drainage area of
the basin which rnay include alluvial plains without any net
erosion or deposition (Fig. ll.la). Similarly, the area of
actual deposition may be larger than a water-filled basin
when additional sediment accumulates on an alluvial plain.
Prograding deltas, particularly in lakes at the foot of mountains, may absorb the majority of incoming sediment and
change the Ar/Ab ratio with time (cf. Section 11.5.2). If
delta prograding cannot be ascertained directly, the mean
sedimentation rate of a basin must be determined from the
(measured or estimated) sediment yield of the entering
rivers and the sedimentation rate in the basin center. Windblown material, as far as it leaves or enters the study area,
is neglected in this method. In most cases the contribution
of eolian sediment to the total sediment budget of various
basins is of minor importance.
A mass balance of terrigenous sediments deposited in
sedimentologically closed water-filled basins may
also aid in evaluating the size of a former source
area. According to equation 11.2 the paleo-drainage
area IS
(11.2b)
To apply this equation, an estimate for D~e is necessary.
The dissolved river load provides elements for
biota producing biogenic sediment and matter for the
precipitation of evaporites in the corresponding basin. For hydrologically open systems the loss of dissolved material, Mlo,t from the basin fill to its underground or to other regions can be calculated ..
Chapter 11 Sediment Supply, Subsidence, and Basin Fill
(11.3)
For this purpose, the masses of mechanical and
biogenic sediments, M",e, Mbi, accumulated in a
closed or semi-c1osed basin during a certain time period must be known. In addition, the ratio D~/DR:h
(= weathering ratio) in the drainage area has to be
measured. Influx from or exchange of dissolved species with an adjacent water body via an outlet is excluded in this case. Under these conditions equation
11.3 is useful in studies of evaporites which may
have been partially dissolved after deposition (cf.
Sect. 11.4). If there is an exchange of dissolved matter with a neighboring water body, as for example
known from marine lagoons, mass balances for
biogenic sediments and evaporites based on chemical
denudation in the drainage area of the basin become
problematic.
11.2.2 Simple Steady-State Models,
Clastic Sediments
First, some simple models may demonstrate the influence of both size and specific sediment yield of various drainage areas on the filling of their corresponding sedimentary basins. The models in Fig. 11.2 are
based on the following assumptions:
- The sizes of the drainage and basin areas are constant (similar to the geometrie basin-filling model,
Seet. 11.1).
- The initial elevation of the basin floor eorresponds
with the sea level whieh is held eonstant.
- The sediments are distributed evenly over the entire
basin.
- The subsidenee rate of all model basins has a medium value (50 mlMa; cf. Chap. 8) and is constant
over the entire basin floor through time.
- The time period eonsidered for denudation and
sediment accumulation is 10 Ma.
Over-Supplied Basins
If the drainage area, A d , on land is large compared
with the area ofthe basin, Ab' for example AiAb=10,
the basin tends to be over-supplied with sediment
and remains filled at all times, unless basin subsidence is very rapid. In Fig. 11.2a and d, over-supply
is true for denudation rates >5 mlMa, beeause sub sidence is assumed to oceur at a rate of 50 mlMa. In
these cases, the basin fills eonsist entirely of eontinental deposits, mostly fluvial sediments. Surplus
river load not used for basin filling is transported
downstream into other depositional areas. This is a
eommon situation in many graben-like structures on
the continents.
- For TE=100%, the mean sedimentation rate, S~e'
of terrigenous sediment in the basin is a function of
D~e and proportional to the ratio of the drainage
area and the basin area, Ai Ab:
(11.1 )
This simple equation is only valid if the bulk densities of both the eroded rocks and the accumulated
sediments (compacted solid material) are equal. To
find the actual sedimentation rate of porous sediment
the conversion factors listed in Fig. 11.1 b have to be
used.
- Vice versa, the mean mechanical denudation rate in
the source area is
(l1.2a)
D~e may be used as a proxy for the reconstruction
of the topography (relief) and climate of a paleodrainage area.
This concept can be used for systems where the sizes of
both the source area and the basin area are known, as for
modem and geologically young denudation-accumulation
systems. In some cases, the source area of sediment (e.g. a
mountain range) is not identical with the drainage area of
the basin which rnay include alluvial plains without any net
erosion or deposition (Fig. ll.la). Similarly, the area of
actual deposition may be larger than a water-filled basin
when additional sediment accumulates on an alluvial plain.
Prograding deltas, particularly in lakes at the foot of mountains, may absorb the majority of incoming sediment and
change the Ar/Ab ratio with time (cf. Section 11.5.2). If
delta prograding cannot be ascertained directly, the mean
sedimentation rate of a basin must be determined from the
(measured or estimated) sediment yield of the entering
rivers and the sedimentation rate in the basin center. Windblown material, as far as it leaves or enters the study area,
is neglected in this method. In most cases the contribution
of eolian sediment to the total sediment budget of various
basins is of minor importance.
A mass balance of terrigenous sediments deposited in
sedimentologically closed water-filled basins may
also aid in evaluating the size of a former source
area. According to equation 11.2 the paleo-drainage
area IS
(11.2b)
To apply this equation, an estimate for D~e is necessary.
The dissolved river load provides elements for
biota producing biogenic sediment and matter for the
precipitation of evaporites in the corresponding basin. For hydrologically open systems the loss of dissolved material, Mlo,t from the basin fill to its underground or to other regions can be calculated ..
Chapter 11 Sediment Supply, Subsidence, and Basin Fill
(11.3)
For this purpose, the masses of mechanical and
biogenic sediments, M",e, Mbi, accumulated in a
closed or semi-c1osed basin during a certain time period must be known. In addition, the ratio D~/DR:h
(= weathering ratio) in the drainage area has to be
measured. Influx from or exchange of dissolved species with an adjacent water body via an outlet is excluded in this case. Under these conditions equation
11.3 is useful in studies of evaporites which may
have been partially dissolved after deposition (cf.
Sect. 11.4). If there is an exchange of dissolved matter with a neighboring water body, as for example
known from marine lagoons, mass balances for
biogenic sediments and evaporites based on chemical
denudation in the drainage area of the basin become
problematic.
11.2.2 Simple Steady-State Models,
Clastic Sediments
First, some simple models may demonstrate the influence of both size and specific sediment yield of various drainage areas on the filling of their corresponding sedimentary basins. The models in Fig. 11.2 are
based on the following assumptions:
- The sizes of the drainage and basin areas are constant (similar to the geometrie basin-filling model,
Seet. 11.1).
- The initial elevation of the basin floor eorresponds
with the sea level whieh is held eonstant.
- The sediments are distributed evenly over the entire
basin.
- The subsidenee rate of all model basins has a medium value (50 mlMa; cf. Chap. 8) and is constant
over the entire basin floor through time.
- The time period eonsidered for denudation and
sediment accumulation is 10 Ma.
Over-Supplied Basins
If the drainage area, A d , on land is large compared
with the area ofthe basin, Ab' for example AiAb=10,
the basin tends to be over-supplied with sediment
and remains filled at all times, unless basin subsidence is very rapid. In Fig. 11.2a and d, over-supply
is true for denudation rates >5 mlMa, beeause sub sidence is assumed to oceur at a rate of 50 mlMa. In
these cases, the basin fills eonsist entirely of eontinental deposits, mostly fluvial sediments. Surplus
river load not used for basin filling is transported
downstream into other depositional areas. This is a
eommon situation in many graben-like structures on
the continents.