11.5 Clastic Sediment, Distribution
This is also one of the assumptions (diffusion algorithms)
used for computer programs simulating sediment accumulation and stratigraphie architecture in basin cross-sections,
but this assumption is only true of speciflc cases. As known
from many modern examples, the directions of sediment
transport in aquatic systems often differ from the straight
land-sea relationship. In studies on ancient basin flUs, sole
marks and other sedimentary structures can reveal the former current systems (paleocurrent analysis). This method is
useful for some depositional environments (e.g. deep-sea
fans), but it is problematic in regions of frequently changing current directions (e.g. in shaUow seas).
.
A straightforward, more or less exponential decrease
in sediment supply can be observed in lakes where
little sediment redistribution takes place (Fig.
11.23a). However, the site of most rapid deposition
below the delta front migrates toward the far end of
the basin with time. Alpine and perialpine lakes,
which receive a great proportion of sandy and
coarser grained bed load, are mainly filled by
prograding delta sediments. These often make up 50
to 90% of the total basin fill.
In shallow, low to medium energy marine basins
(epicontinental seas), the basin floor may maintain a
certain equilibrium profile generated by the hydrodynamic regime ofthe basin (Fig. 11.23b).
When more sediment is supplied than can be accommodated by subsidence, the surplus sediment is transported into deeper water (sediment bypassing). As a
result, the sediment thicknesses deposited during a
certain time unit are mainly controlled by differential
subsidence. Long-term sedimentation rates in areas
elose to the coastline therefore tend to be slow,
whereas the depocenter with the highest sedimentation rate may be located far away from the coast.
Shelf-slope-deep sea settings of high energy and
differential subsidence (Fig. 11.23c) exhibit a behavior similar to that of shallow-marine basins. The site
of most rapid deposition here is the slope or a deepsea fan fed by turbidites. Even deep-sea sediments
can be frequently redistributed by bottom currents
(cf. Sect. 5.5).
The models in Fig. 11.23b,c demonstrate that sediment distribution in common marine environments
does not follow a simple rule. In addition, relative
lake or sea-Ievel changes may considerably modify
the local or regional situation (cf. Sects. 7.3 and 7.4).
These complications are only briefly mentioned here.
Some further problems will be addressed below by a
number of modern examples dealing with the transfer
of river sediments to deltas, prograding coastlines,
and deep-sea fans.
11.5.2 Transfer ofRiver Sediments to
Marine Deltas
Out- and Upbuilding of Sediment from a
Point Source
519
Given a point source (river mouth) with large and
constant input of sediment into a deepening basin,
the pro gradation of the deltaic sediment wedge slows
through time (Fig. 11.24a,b). This results from two
factors: (1) increasingly more sediment is required to
form fore sets of the same thickness on the inelined
basin floor; (2) more sediment is needed with time to
build out a three-dimensional sediment body at a
constant rate. Falling sea level causes accelerated
seaward pro gradation, while rising sea level may
result either in coastal retreat or continued, but reduced seaward progradation, depending on the rates
of both sea-Ievel rise and sediment supply (Fig.
11.24c and d). Further influences of sea-Ievel
changes on deltaic systems are discussed in Sect. 7.4.
The main question addressed in the following exampIes is: what proportion of the incoming river load is
used up for the prograding of deltas and wider coastlines, and how much sediment is transferred to
deeper and more remote parts ofthe basin?
Delta Prograding into Low- to Medium-Energy
Marine Basins
The Holocene was aperiod in which marine deltas
began to prograde seaward due to the deceleration of
sea-Ievel rise after the last glacial lowstand (Stanley
and Warne 1994). The following examples are taken
from this time period.
Several medium to major rivers enter the Mediterranean Sea and generate prograding, more or less
wave-dominated or mixed river- and wave-dominated
lobate deltas, such as the deltas of the rivers Nile,
Rhöne, Ebro and others (cf. Sect. 3.5). The tidal
range of the semi-elosed Mediterranean is very low
and wind-generated waves and currents are normally
of low to medium energy. Similarly, the prograding
of the Danube delta into the Black Sea and that of the
Volga delta into the Caspian Sea is moderately affected by waves and currents. Here, some further,
well-studied examples are described.
Po delta and Tiber delta, Italy. With the aid ofhistorical marks, the late Holocene development of
these deltas and their adjacent coastlines has been
established. This allows an estimate of the partition
of the river-borne sediment into portions stored either
in the delta area or deposited in deeper water.
ThePo River drains part of the Alps and, to a minor degree, the Apennines and builds a delta out into the shaUow
northern Adriatic Sea (Fig. 11.25a). Using the weU-dated
This is also one of the assumptions (diffusion algorithms)
used for computer programs simulating sediment accumulation and stratigraphie architecture in basin cross-sections,
but this assumption is only true of speciflc cases. As known
from many modern examples, the directions of sediment
transport in aquatic systems often differ from the straight
land-sea relationship. In studies on ancient basin flUs, sole
marks and other sedimentary structures can reveal the former current systems (paleocurrent analysis). This method is
useful for some depositional environments (e.g. deep-sea
fans), but it is problematic in regions of frequently changing current directions (e.g. in shaUow seas).
.
A straightforward, more or less exponential decrease
in sediment supply can be observed in lakes where
little sediment redistribution takes place (Fig.
11.23a). However, the site of most rapid deposition
below the delta front migrates toward the far end of
the basin with time. Alpine and perialpine lakes,
which receive a great proportion of sandy and
coarser grained bed load, are mainly filled by
prograding delta sediments. These often make up 50
to 90% of the total basin fill.
In shallow, low to medium energy marine basins
(epicontinental seas), the basin floor may maintain a
certain equilibrium profile generated by the hydrodynamic regime ofthe basin (Fig. 11.23b).
When more sediment is supplied than can be accommodated by subsidence, the surplus sediment is transported into deeper water (sediment bypassing). As a
result, the sediment thicknesses deposited during a
certain time unit are mainly controlled by differential
subsidence. Long-term sedimentation rates in areas
elose to the coastline therefore tend to be slow,
whereas the depocenter with the highest sedimentation rate may be located far away from the coast.
Shelf-slope-deep sea settings of high energy and
differential subsidence (Fig. 11.23c) exhibit a behavior similar to that of shallow-marine basins. The site
of most rapid deposition here is the slope or a deepsea fan fed by turbidites. Even deep-sea sediments
can be frequently redistributed by bottom currents
(cf. Sect. 5.5).
The models in Fig. 11.23b,c demonstrate that sediment distribution in common marine environments
does not follow a simple rule. In addition, relative
lake or sea-Ievel changes may considerably modify
the local or regional situation (cf. Sects. 7.3 and 7.4).
These complications are only briefly mentioned here.
Some further problems will be addressed below by a
number of modern examples dealing with the transfer
of river sediments to deltas, prograding coastlines,
and deep-sea fans.
11.5.2 Transfer ofRiver Sediments to
Marine Deltas
Out- and Upbuilding of Sediment from a
Point Source
519
Given a point source (river mouth) with large and
constant input of sediment into a deepening basin,
the pro gradation of the deltaic sediment wedge slows
through time (Fig. 11.24a,b). This results from two
factors: (1) increasingly more sediment is required to
form fore sets of the same thickness on the inelined
basin floor; (2) more sediment is needed with time to
build out a three-dimensional sediment body at a
constant rate. Falling sea level causes accelerated
seaward pro gradation, while rising sea level may
result either in coastal retreat or continued, but reduced seaward progradation, depending on the rates
of both sea-Ievel rise and sediment supply (Fig.
11.24c and d). Further influences of sea-Ievel
changes on deltaic systems are discussed in Sect. 7.4.
The main question addressed in the following exampIes is: what proportion of the incoming river load is
used up for the prograding of deltas and wider coastlines, and how much sediment is transferred to
deeper and more remote parts ofthe basin?
Delta Prograding into Low- to Medium-Energy
Marine Basins
The Holocene was aperiod in which marine deltas
began to prograde seaward due to the deceleration of
sea-Ievel rise after the last glacial lowstand (Stanley
and Warne 1994). The following examples are taken
from this time period.
Several medium to major rivers enter the Mediterranean Sea and generate prograding, more or less
wave-dominated or mixed river- and wave-dominated
lobate deltas, such as the deltas of the rivers Nile,
Rhöne, Ebro and others (cf. Sect. 3.5). The tidal
range of the semi-elosed Mediterranean is very low
and wind-generated waves and currents are normally
of low to medium energy. Similarly, the prograding
of the Danube delta into the Black Sea and that of the
Volga delta into the Caspian Sea is moderately affected by waves and currents. Here, some further,
well-studied examples are described.
Po delta and Tiber delta, Italy. With the aid ofhistorical marks, the late Holocene development of
these deltas and their adjacent coastlines has been
established. This allows an estimate of the partition
of the river-borne sediment into portions stored either
in the delta area or deposited in deeper water.
ThePo River drains part of the Alps and, to a minor degree, the Apennines and builds a delta out into the shaUow
northern Adriatic Sea (Fig. 11.25a). Using the weU-dated