11.2 Denudation-Sediment Accumulation Systems
to 1 mika. The highest rates were found in prodelta areas
characterized by very gently inc1ined foresets and proximal
bottomsets. All these rates appear to be in good agreement
with the amount of suspended load which the rivers discharge into the sea (Yellow River on the order of 1 x 10 9
tla). Under steady-state conditions, but with a rate of subsidence lower than the mean sedimentation rate, the Yellow
Sea will become filled up with sediments in a very short
geological time period. The effects of land use in the huge
loess region of northem central China are not considered
here.
Basins with Moderate to Low Sediment Supply
Given a ratio of Ai Ab ~ 1, the denudation rate on
land predominantly controls whether or not the basin
will be filled with continental or aquatic sediments.
High and medium denudation rates (Fig. 11.2b) still
deliver more sediment volume than required for basin
filling due to moderate subsidence (50 m1Ma). A low
denudation rate (Fig. 11.2e) only leads to insufficient
sediment supply and thus to a deepening basin with
lake or marine sediments. In this case, half of the
sediment may be autochthonous, provided the
biogenie sediment production reaches a medium
value of 10 mlMa,
With decreasing Ad/A b (= 0.1 in Fig. 11.2c and f),
even a high rate of denudation is not able to fill the
basin and compensate for subsidence. To get equilibrium between subsidence and sediment buildup (50
mlMa) a denudation rate of 500 mlMa is necessary in
this case. Under the assumptions of (c) only one half
of the resulting marine sediment is terrigenous which
is typical of hemipelagic sediments in the modern
oceans. Adl Ab =0.1 in combination with low denudation eventually leads to predominantly bio genie marine sediments containing only a small fraction of
land-derived material. This model approximates the
conditions of pelagic sedimentation in large ocean
basins some distance away from a continent (see below).
Influence of Several Sediment Sources
The influence of various terrigenous sediment
sources on basin fills is shown qualitatively in Fig.
II.3a. Rivers flowing through lakes before they
reach the sea deposit their bed load and suspended
load in the lake and therefore carry little material into
the sea. A high-relief mountain range bordering the
sea may exert a strong influence in terms of sedimentation rate, texture, and composition on the filling of
a neighboring basin, even if a medium to major river
from a large hinterland enters the same basin. Lowrelief peninsulas and islands of limited extension
commonly shed small amounts of clastic material
into the sea compared with major rivers draining
487
large areas of differing relief and climate. For this
reason, the existence of islands in large, ancient
ocean basins is often difficult to assess from the basin fill, unless the island differs significantly in its
petrographie characteristics from the other, volumetrically predominating sediment sourees.
Sediment budgets of some closed and half-closed
basins are described below, those of accretionary
wedges are discussed in Section 12.5.
Transition to Dynamic Models
If at least one of the factors controlling basin filling
changes during basin evolution, the steady-state system defined above is transformed into adynamie system. This occurs, for example when the relief in the
drainage area is enhanced with time by tectonic uplift. This will lead to strongly increased input of
terrigenous material (cf. Sect. 9.3) and thus to a
shallowing and ultimately fluvial depositional environment (e.g. for AiAb=l; Fig. 11.2e). Simultaneously, the proportion of terrigenous material will
increase at the expense of bio genie components (as,
e.g., on the Texas-Louisiana shelf, cf. Fig. 11.32a-c).
On the contrary, a lowering of relief in the denudation area diminishes the ratio of terrigenous and
bio genie sediment components and causes basin
deepening, provided the subsidence rate remains constant.
11.2.3 Modern Examples of Steady-State
DA Systems
Considering short time periods of some thousand
years (e.g. in the Holocene) changes in the characteristics of the drainage area (size, relief, climate, etc.)
and basin area (size, subsidence, etc.) can be largely
neglected. Such systems are here referred to as
"steady-state" systems in which sediment yield from
land areas and the mean sedimentation rate of terrestrial material do not significantly change with time.
U nder favorable conditions (e.g. limited erosion and
slow basin floor subsidence) steady-state conditions
mayaiso be assumed for some millions of years. Examples of quasi-steady-state systems are young lake
basins and some larger basins located either on continental ernst or adjacent to great ocean basins.
Open Lake Systems
Lake basins originate from different processes (Sect.
2.5). Many of them are small and display negligible
subsidence of their basin floor in relation to the surrounding land areas and compared with the high sedimentation rates in the lakes (often > 1 mmJa). Such
to 1 mika. The highest rates were found in prodelta areas
characterized by very gently inc1ined foresets and proximal
bottomsets. All these rates appear to be in good agreement
with the amount of suspended load which the rivers discharge into the sea (Yellow River on the order of 1 x 10 9
tla). Under steady-state conditions, but with a rate of subsidence lower than the mean sedimentation rate, the Yellow
Sea will become filled up with sediments in a very short
geological time period. The effects of land use in the huge
loess region of northem central China are not considered
here.
Basins with Moderate to Low Sediment Supply
Given a ratio of Ai Ab ~ 1, the denudation rate on
land predominantly controls whether or not the basin
will be filled with continental or aquatic sediments.
High and medium denudation rates (Fig. 11.2b) still
deliver more sediment volume than required for basin
filling due to moderate subsidence (50 m1Ma). A low
denudation rate (Fig. 11.2e) only leads to insufficient
sediment supply and thus to a deepening basin with
lake or marine sediments. In this case, half of the
sediment may be autochthonous, provided the
biogenie sediment production reaches a medium
value of 10 mlMa,
With decreasing Ad/A b (= 0.1 in Fig. 11.2c and f),
even a high rate of denudation is not able to fill the
basin and compensate for subsidence. To get equilibrium between subsidence and sediment buildup (50
mlMa) a denudation rate of 500 mlMa is necessary in
this case. Under the assumptions of (c) only one half
of the resulting marine sediment is terrigenous which
is typical of hemipelagic sediments in the modern
oceans. Adl Ab =0.1 in combination with low denudation eventually leads to predominantly bio genie marine sediments containing only a small fraction of
land-derived material. This model approximates the
conditions of pelagic sedimentation in large ocean
basins some distance away from a continent (see below).
Influence of Several Sediment Sources
The influence of various terrigenous sediment
sources on basin fills is shown qualitatively in Fig.
II.3a. Rivers flowing through lakes before they
reach the sea deposit their bed load and suspended
load in the lake and therefore carry little material into
the sea. A high-relief mountain range bordering the
sea may exert a strong influence in terms of sedimentation rate, texture, and composition on the filling of
a neighboring basin, even if a medium to major river
from a large hinterland enters the same basin. Lowrelief peninsulas and islands of limited extension
commonly shed small amounts of clastic material
into the sea compared with major rivers draining
487
large areas of differing relief and climate. For this
reason, the existence of islands in large, ancient
ocean basins is often difficult to assess from the basin fill, unless the island differs significantly in its
petrographie characteristics from the other, volumetrically predominating sediment sourees.
Sediment budgets of some closed and half-closed
basins are described below, those of accretionary
wedges are discussed in Section 12.5.
Transition to Dynamic Models
If at least one of the factors controlling basin filling
changes during basin evolution, the steady-state system defined above is transformed into adynamie system. This occurs, for example when the relief in the
drainage area is enhanced with time by tectonic uplift. This will lead to strongly increased input of
terrigenous material (cf. Sect. 9.3) and thus to a
shallowing and ultimately fluvial depositional environment (e.g. for AiAb=l; Fig. 11.2e). Simultaneously, the proportion of terrigenous material will
increase at the expense of bio genie components (as,
e.g., on the Texas-Louisiana shelf, cf. Fig. 11.32a-c).
On the contrary, a lowering of relief in the denudation area diminishes the ratio of terrigenous and
bio genie sediment components and causes basin
deepening, provided the subsidence rate remains constant.
11.2.3 Modern Examples of Steady-State
DA Systems
Considering short time periods of some thousand
years (e.g. in the Holocene) changes in the characteristics of the drainage area (size, relief, climate, etc.)
and basin area (size, subsidence, etc.) can be largely
neglected. Such systems are here referred to as
"steady-state" systems in which sediment yield from
land areas and the mean sedimentation rate of terrestrial material do not significantly change with time.
U nder favorable conditions (e.g. limited erosion and
slow basin floor subsidence) steady-state conditions
mayaiso be assumed for some millions of years. Examples of quasi-steady-state systems are young lake
basins and some larger basins located either on continental ernst or adjacent to great ocean basins.
Open Lake Systems
Lake basins originate from different processes (Sect.
2.5). Many of them are small and display negligible
subsidence of their basin floor in relation to the surrounding land areas and compared with the high sedimentation rates in the lakes (often > 1 mmJa). Such