11.6 Consequenees for Stratigraphie Sequenees
11.5.9 Summary (Sediment Distribution)
- The assumption of an exponential decrease in
the sedimentation rate from a point source toward the morphological center of a water-filled
basin is only valid for some specific cases.
- In medium- to high-energy shallow-marine basins, sediment bypassing is common; even in
deep basins great proportions of sediment may
be redistributed by bottom currents. As a result,
sediment thicknesses are largely controlled by
the hydrodynamic regime of the basin and differential subsidence.
- Holocene (sea-Ievel highstand) up- and outbuilding of delta plains and prodelta slopes into
low- to medium-energy marine environments
has often occurred at rates of several m/a, including prograding of adjacent coastlines. This
process has consumed at least one half of the
incoming river sediment. The same applies to
some lobate marine deltas subjected to high er
energy conditions. The same degree of sediment partition mayaiso be valid for longer time
periods. The (upper) prodelta slope is the site of
most rapid deposition.
11.6 Consequenees for Stratigraphie
Sequenees and Facies Associations
(Overview)
11.6.1 Vertical Facies Evolution:
Three Principal Types
Summarizing the results of the interplay between
denudation and depositional areas, sedimentation,
and subsidence, three principal alternatives for the
vertical facies evolution in sedimentary basins can be
distinguished:
(1) Vertical sediment buildup ~ subsidence (oversupplied basins): shallowing basin, transition from
marine or lake deposits into fluvial or eolian sediments. Decrease in the proportion of biogenic sediment components. In the case of carbonates: increase
in benthic and reef-derived carbonate.
(2) Sediment buildup ::; subsidence (sedimentstarved conditions): deepening basin, transition from
continental or shallow-marine into deep-marine environment. Increase in the proportion of biogenic sediment components, except for water depths below the
lysocline (dissolution of carbonate and opaline silica).
(3) Sediment supply and vertical buildup approximately compensates for subsidence ("balanced" basins): long-persisting identical morphological basin
533
- The prograding of barrier-Iagoon systems
along high-energy co asts seems to be largely
fed by longshore sediment transport or material
derived from the shallow sea. This is shown
for Holocene and late Pleistocene examples.
- The transfer of river sediment to deep-sea fans
varies widely (transfer ratios from <10 to
100%). Fluvial-dominated and lobate deltas
tend to convey a lower proportion of the incoming sediment to the deep sea than tidedominated deltas. The latter also transfer sediment to fans during sea-Ievel highstands. The
long-term sediment transfer is essentially controlled by terrigenous sediment supply and is
little affected by sea-Ievel changes.
- Apart from deltas, prodeltas, and deep-sea fans
(which may migrate), a large proportion of the
river-borne sediment is widely dispersed on
shelves and in deeper basins. Along passive
continental margins, terrigenous sediments are
preferentially deposited on the outer shelf, continental slope and upper continental rise.
configuration, maintenance of deep-marine, shallowmarine, or continental facies. No long-term drastic
change in the proportions of terrigenous and biogenic
sediment components.
Basins Over-Supplied with Sediment
Over-supplied basins are short-lived, but well represented in the geologic record. Their facies types are
dominated by clastic sediments and show a shallowing-upward trend. This category of basins comprises
deep-sea to continental basin settings and environments, including
- Graben structures and rift basins with fluvial deposits (examples of Sect. 12.1).
- Lakes fed by rivers from high-relief regions (Sects.
2.2.2 and 2.5.1) and proglacial lake deposits (Sect.
2.1.1 and Sect. 11.2).
- Marine deltas and deep-sea fans (Sects. 3.5, 5.4.2,
and 11.5).
- Rernnant basins with turbidites (Sect. 12.6.1).
- Foreland basins with flysch and molasse deposition
(Sect. 12.6.2).
- Forearc and backarc basins with high supply of
volcaniclastics and other clastic material (Sects. 12
5.3 and 12.5.4).
11.5.9 Summary (Sediment Distribution)
- The assumption of an exponential decrease in
the sedimentation rate from a point source toward the morphological center of a water-filled
basin is only valid for some specific cases.
- In medium- to high-energy shallow-marine basins, sediment bypassing is common; even in
deep basins great proportions of sediment may
be redistributed by bottom currents. As a result,
sediment thicknesses are largely controlled by
the hydrodynamic regime of the basin and differential subsidence.
- Holocene (sea-Ievel highstand) up- and outbuilding of delta plains and prodelta slopes into
low- to medium-energy marine environments
has often occurred at rates of several m/a, including prograding of adjacent coastlines. This
process has consumed at least one half of the
incoming river sediment. The same applies to
some lobate marine deltas subjected to high er
energy conditions. The same degree of sediment partition mayaiso be valid for longer time
periods. The (upper) prodelta slope is the site of
most rapid deposition.
11.6 Consequenees for Stratigraphie
Sequenees and Facies Associations
(Overview)
11.6.1 Vertical Facies Evolution:
Three Principal Types
Summarizing the results of the interplay between
denudation and depositional areas, sedimentation,
and subsidence, three principal alternatives for the
vertical facies evolution in sedimentary basins can be
distinguished:
(1) Vertical sediment buildup ~ subsidence (oversupplied basins): shallowing basin, transition from
marine or lake deposits into fluvial or eolian sediments. Decrease in the proportion of biogenic sediment components. In the case of carbonates: increase
in benthic and reef-derived carbonate.
(2) Sediment buildup ::; subsidence (sedimentstarved conditions): deepening basin, transition from
continental or shallow-marine into deep-marine environment. Increase in the proportion of biogenic sediment components, except for water depths below the
lysocline (dissolution of carbonate and opaline silica).
(3) Sediment supply and vertical buildup approximately compensates for subsidence ("balanced" basins): long-persisting identical morphological basin
533
- The prograding of barrier-Iagoon systems
along high-energy co asts seems to be largely
fed by longshore sediment transport or material
derived from the shallow sea. This is shown
for Holocene and late Pleistocene examples.
- The transfer of river sediment to deep-sea fans
varies widely (transfer ratios from <10 to
100%). Fluvial-dominated and lobate deltas
tend to convey a lower proportion of the incoming sediment to the deep sea than tidedominated deltas. The latter also transfer sediment to fans during sea-Ievel highstands. The
long-term sediment transfer is essentially controlled by terrigenous sediment supply and is
little affected by sea-Ievel changes.
- Apart from deltas, prodeltas, and deep-sea fans
(which may migrate), a large proportion of the
river-borne sediment is widely dispersed on
shelves and in deeper basins. Along passive
continental margins, terrigenous sediments are
preferentially deposited on the outer shelf, continental slope and upper continental rise.
configuration, maintenance of deep-marine, shallowmarine, or continental facies. No long-term drastic
change in the proportions of terrigenous and biogenic
sediment components.
Basins Over-Supplied with Sediment
Over-supplied basins are short-lived, but well represented in the geologic record. Their facies types are
dominated by clastic sediments and show a shallowing-upward trend. This category of basins comprises
deep-sea to continental basin settings and environments, including
- Graben structures and rift basins with fluvial deposits (examples of Sect. 12.1).
- Lakes fed by rivers from high-relief regions (Sects.
2.2.2 and 2.5.1) and proglacial lake deposits (Sect.
2.1.1 and Sect. 11.2).
- Marine deltas and deep-sea fans (Sects. 3.5, 5.4.2,
and 11.5).
- Rernnant basins with turbidites (Sect. 12.6.1).
- Foreland basins with flysch and molasse deposition
(Sect. 12.6.2).
- Forearc and backarc basins with high supply of
volcaniclastics and other clastic material (Sects. 12
5.3 and 12.5.4).