512
eastern shelf area. Consequently, the sedimentation
rate decreased during the past 4 Ma as is characteristic of an extensional basin. Applying a mean ratio of
Ai Ab = 1.4, an average denudation rate of about 20
rnIMa for the past 4 Ma was determined. If the sediment vo1ume is only related to the mountain ranges,
the denudation rate becomes higher (about 50 rnIMa).
The limited drainage area and its relatively low average denudation rate as well as rapidly operating tectonic movements explain why the Gulf of California
is still a deep ocean basin.
11.3.7 Summary (DA Basin Models)
The interplay between the source areas, A d , of
terrestrial sediment and various basin types (area
Ab) and their subsidence history is demonstrated
by a number of simplistic model basins.
- Clastic basin filling is largely controlled by the
Ai Ab ratio and the mean rate of mechanical denudation, D~e'
In extensional basins, the mean mechanical sedimentation rate, S~e, decreases with time, in
closing basins it increases (Fig. 11.20a). The
filling of foreland basins is accentuated by
pulses of overthrusting (Fig. 11.20b, land 11)
and drastic changes in sediment input and Sr me .
11.4 Chemical Sediments (Evaporites)
in Basin Filling
11.4.1 General Aspects
Chemical sediments or evaporites play a significant
part in the filling of rnany basins. They form either
by evaporation of sea water in coastal lagoons and
larger basins connected with the ocean, or they occur
in closed lake basins fed by river water. Marine salts
usually accumulate rapidly and generate thick sequences in very short time intervals (cf. Sect. 6.4). A
mass balance for these evaporites, which includes the
sediment source, is not possible, however, because
the salt reservoir of the oceans is practically unlimited.
The situation is different for closed lake basins
with certain drainage areas which deliver dissolved
material released from various rock types by weathering (cf. Sect. 9.2). Provided the basin floor is impermeable (which is not always the case), then the
incorning dissolved matter remains entirely in the
lake basin and can form evaporites. Therefore it is
possible to establish mass balances for modern lakes,
the source areas of which are known, similar to the
balances discussed above for terrigenous sediments.
Some of the problems and results of this approach
Cbapter 11 Sediment Supply, Subsidence, and Basin Fill
Part of the mountain-derived material was deposited on
alluvial plains before it reached the gulf, but even the denudation rate calculated for the mountainous area appears to
be rather low. The drainage area ofthe gulflies in the transition zone between the hot dry c1imate to the northwest
and the semiarid and increasingly wet c1irnate of the lowlands and mountain ranges of mainland Mexico, the Sierra
Madre Occidental, in the southeast. Here, some rivers drain
regions of high relief, consisting largely of Cenozoic
volcanics. It can be assumed that the size of the drainage
area has not changed significantly during the last 4 Ma.
The models predict periods of deepening and
shallowing basins as well as transitions from
continental to aquatic environments and vice
versa.
Simplistic models of entire basins cannot account for various complications realized in nature, such as differential subsidence and nonuniform sediment distribution, or lateral facies
changes within the basin (two- and three-dimensional effects). Nevertheless, such models
can help to better understand the evolution of
more complex basins.
are briefly mentioned here. Biogenic sediment production is neglected. A general overview of the sediments of closed lake basins is given in Sect. 2.5.
11.4.2 Mass Balances of Closed Lake Systems
(One Rock Type)
For simplified chernical budgets of lakes, it is assumed that the dissolved constituents are precipitated
inorganically and form solid, pore-free layers in lake
basins. These models simulate to some extent the
situation in salt lakes (Sect. 2.5.1) which are, however, not devoid of biota and terrigenous material.
Some results of such a model are shown in Fig. 11.21
for three different types of rock exposed to weathering in the drainage area of the lake.
In the model in Fig.l1.21 thedepositional area of the lake
remains constant and makes up one tenth of the denudation
area feeding the lake with river water. Water depth and
subsidence of the lake basin floor are sufficient to take up
sediments during a time interval of at least I Ma. The climate in the drainage area of the lake is predominantly
semiarid to humid, in the lake area itself it is arid. The rock
types in the denudation area may be either granitic, basaltic, or carbonates. The average chemical denudation
rates are assumed to be on the order of 5, 10, and 30 m/Ma,
eastern shelf area. Consequently, the sedimentation
rate decreased during the past 4 Ma as is characteristic of an extensional basin. Applying a mean ratio of
Ai Ab = 1.4, an average denudation rate of about 20
rnIMa for the past 4 Ma was determined. If the sediment vo1ume is only related to the mountain ranges,
the denudation rate becomes higher (about 50 rnIMa).
The limited drainage area and its relatively low average denudation rate as well as rapidly operating tectonic movements explain why the Gulf of California
is still a deep ocean basin.
11.3.7 Summary (DA Basin Models)
The interplay between the source areas, A d , of
terrestrial sediment and various basin types (area
Ab) and their subsidence history is demonstrated
by a number of simplistic model basins.
- Clastic basin filling is largely controlled by the
Ai Ab ratio and the mean rate of mechanical denudation, D~e'
In extensional basins, the mean mechanical sedimentation rate, S~e, decreases with time, in
closing basins it increases (Fig. 11.20a). The
filling of foreland basins is accentuated by
pulses of overthrusting (Fig. 11.20b, land 11)
and drastic changes in sediment input and Sr me .
11.4 Chemical Sediments (Evaporites)
in Basin Filling
11.4.1 General Aspects
Chemical sediments or evaporites play a significant
part in the filling of rnany basins. They form either
by evaporation of sea water in coastal lagoons and
larger basins connected with the ocean, or they occur
in closed lake basins fed by river water. Marine salts
usually accumulate rapidly and generate thick sequences in very short time intervals (cf. Sect. 6.4). A
mass balance for these evaporites, which includes the
sediment source, is not possible, however, because
the salt reservoir of the oceans is practically unlimited.
The situation is different for closed lake basins
with certain drainage areas which deliver dissolved
material released from various rock types by weathering (cf. Sect. 9.2). Provided the basin floor is impermeable (which is not always the case), then the
incorning dissolved matter remains entirely in the
lake basin and can form evaporites. Therefore it is
possible to establish mass balances for modern lakes,
the source areas of which are known, similar to the
balances discussed above for terrigenous sediments.
Some of the problems and results of this approach
Cbapter 11 Sediment Supply, Subsidence, and Basin Fill
Part of the mountain-derived material was deposited on
alluvial plains before it reached the gulf, but even the denudation rate calculated for the mountainous area appears to
be rather low. The drainage area ofthe gulflies in the transition zone between the hot dry c1imate to the northwest
and the semiarid and increasingly wet c1irnate of the lowlands and mountain ranges of mainland Mexico, the Sierra
Madre Occidental, in the southeast. Here, some rivers drain
regions of high relief, consisting largely of Cenozoic
volcanics. It can be assumed that the size of the drainage
area has not changed significantly during the last 4 Ma.
The models predict periods of deepening and
shallowing basins as well as transitions from
continental to aquatic environments and vice
versa.
Simplistic models of entire basins cannot account for various complications realized in nature, such as differential subsidence and nonuniform sediment distribution, or lateral facies
changes within the basin (two- and three-dimensional effects). Nevertheless, such models
can help to better understand the evolution of
more complex basins.
are briefly mentioned here. Biogenic sediment production is neglected. A general overview of the sediments of closed lake basins is given in Sect. 2.5.
11.4.2 Mass Balances of Closed Lake Systems
(One Rock Type)
For simplified chernical budgets of lakes, it is assumed that the dissolved constituents are precipitated
inorganically and form solid, pore-free layers in lake
basins. These models simulate to some extent the
situation in salt lakes (Sect. 2.5.1) which are, however, not devoid of biota and terrigenous material.
Some results of such a model are shown in Fig. 11.21
for three different types of rock exposed to weathering in the drainage area of the lake.
In the model in Fig.l1.21 thedepositional area of the lake
remains constant and makes up one tenth of the denudation
area feeding the lake with river water. Water depth and
subsidence of the lake basin floor are sufficient to take up
sediments during a time interval of at least I Ma. The climate in the drainage area of the lake is predominantly
semiarid to humid, in the lake area itself it is arid. The rock
types in the denudation area may be either granitic, basaltic, or carbonates. The average chemical denudation
rates are assumed to be on the order of 5, 10, and 30 m/Ma,