3.5 Marine Delta Complexes
3.5.5 Response of Marine Deltas to Sea-Level
Changes
The impact of eustatic sea-Ievel changes on the facies
architecture of marine deltas is demonstrated in Fig.
3.36 for the classicallobate delta type (see also Sect.
7.4.1). A rapid rise in sea level leads to extensive
coastal retreat on top of the submerging former delta
plain. If the sea-Ievel rise does not exceed a few tens
of meters and then persists for some time, the delta
front can pro grade again as a shallow-water delta
(Fig. 3.36a). As soon as the position of the former
delta front is reached, further outbuilding takes place
in deeper water and therefore will proceed more
slowly (Fig. 3.36b, deep-water delta). Falling sea
level prornotes coastal advance in conjunction with
the prograding delta front to a deeper level. The former delta plain emerges above sea level by tens of
meters and will therefore be dissected and partially
eroded by the main river and its tributaries. The
eroded material contributes significantly to the further outbuilding of the lowstand delta. Part of the
emerged deltaic terrace mayaiso be removed by large
slumps and mass flows (Sect. 5.4.1). A subsequent
sea-level rise to its initial level tends to restore the
former coastline and to generate again a transgressive
sequence (Fig. 3.36c, sirnilar to that shown in a).
The vertical facies successions resulting from
such sea-Ievel changes vary from location to location
within the delta complex (Fig. 3.36d through f).
Interdistributary sites landward or seaward of the
initial coastline showalternations between the following facies groups (listed from land to sea):
3.5.6 Summary (Marine Deltas)
Prograding major marine deltas and their sediments cover large areas which exhibit a variety
of terrestrial, coastal, and shallow-marine
depositional environments.
The morphology and sedimentary facies of the
deltas is controlled to a large degree by sediment
supply and the hydrodynarnic regime of the marine basin (fluvial-, wave-, tide-dominated deltas).
165
Lower delta-plain assoclatlOn (lagoonal, tidal,
marsh, crevasse splay deposits) alternating with
the alluvial-plain association (flood plain, point
bar, channel fill, lake deposits, etc.; Fig. 3.36f).
- Lower delta-plain association alternating with
shoreface and delta-front deposits (delta-front association; Fig. 3.36e).
- Shoreface/delta front alternating with the inner
shelf/prodelta association (Fig. 3.36d).
Most of these different facies successions show variations between two principal groups and are restricted to certain parts of the delta complex. Some of
them may, however, occur and follow each other in
the same area and thus display greater facies variations. Other complications may arise from delta-lobe
switching. Point bar deposits, channel fills, and their
associated levee sediments may become stacked due
to alternating periods of downcutting and refilling of
the distributaries (Fig. 3.36a).
When the sea-level fluctuations reach amplitudes
in the order of 50 to 100 m, prodelta sediments and
inner shelf associations playa greater part in the facies successions than described in the model of Fig.
3.36.
An unusually high relative sea-level rise in relation to previous highstands may cause a transgression beyond the
former delta plain a long way upriver. This would completely change the configliration of the depositional area.
Such scenarios are rarely discussed in studies on the effects
of sea-level fluctuations on deltaic sediments. In any case,
the flat lowlands of a delta plain are the prototype of an
environment, where even small sea-level oscillations can
affect the depositional processes of large areas.
- Delta-lobe switching, constructional and destructional phases, and response to eustatic sealevel changes create complex lateral and vertical facies associations, including cyclic sequences.
- Buried peats, growth faults and depobelts in
conjunction with prodelta deposits provide targets for coal and hydrocarbon exploration.
3.5.5 Response of Marine Deltas to Sea-Level
Changes
The impact of eustatic sea-Ievel changes on the facies
architecture of marine deltas is demonstrated in Fig.
3.36 for the classicallobate delta type (see also Sect.
7.4.1). A rapid rise in sea level leads to extensive
coastal retreat on top of the submerging former delta
plain. If the sea-Ievel rise does not exceed a few tens
of meters and then persists for some time, the delta
front can pro grade again as a shallow-water delta
(Fig. 3.36a). As soon as the position of the former
delta front is reached, further outbuilding takes place
in deeper water and therefore will proceed more
slowly (Fig. 3.36b, deep-water delta). Falling sea
level prornotes coastal advance in conjunction with
the prograding delta front to a deeper level. The former delta plain emerges above sea level by tens of
meters and will therefore be dissected and partially
eroded by the main river and its tributaries. The
eroded material contributes significantly to the further outbuilding of the lowstand delta. Part of the
emerged deltaic terrace mayaiso be removed by large
slumps and mass flows (Sect. 5.4.1). A subsequent
sea-level rise to its initial level tends to restore the
former coastline and to generate again a transgressive
sequence (Fig. 3.36c, sirnilar to that shown in a).
The vertical facies successions resulting from
such sea-Ievel changes vary from location to location
within the delta complex (Fig. 3.36d through f).
Interdistributary sites landward or seaward of the
initial coastline showalternations between the following facies groups (listed from land to sea):
3.5.6 Summary (Marine Deltas)
Prograding major marine deltas and their sediments cover large areas which exhibit a variety
of terrestrial, coastal, and shallow-marine
depositional environments.
The morphology and sedimentary facies of the
deltas is controlled to a large degree by sediment
supply and the hydrodynarnic regime of the marine basin (fluvial-, wave-, tide-dominated deltas).
165
Lower delta-plain assoclatlOn (lagoonal, tidal,
marsh, crevasse splay deposits) alternating with
the alluvial-plain association (flood plain, point
bar, channel fill, lake deposits, etc.; Fig. 3.36f).
- Lower delta-plain association alternating with
shoreface and delta-front deposits (delta-front association; Fig. 3.36e).
- Shoreface/delta front alternating with the inner
shelf/prodelta association (Fig. 3.36d).
Most of these different facies successions show variations between two principal groups and are restricted to certain parts of the delta complex. Some of
them may, however, occur and follow each other in
the same area and thus display greater facies variations. Other complications may arise from delta-lobe
switching. Point bar deposits, channel fills, and their
associated levee sediments may become stacked due
to alternating periods of downcutting and refilling of
the distributaries (Fig. 3.36a).
When the sea-level fluctuations reach amplitudes
in the order of 50 to 100 m, prodelta sediments and
inner shelf associations playa greater part in the facies successions than described in the model of Fig.
3.36.
An unusually high relative sea-level rise in relation to previous highstands may cause a transgression beyond the
former delta plain a long way upriver. This would completely change the configliration of the depositional area.
Such scenarios are rarely discussed in studies on the effects
of sea-level fluctuations on deltaic sediments. In any case,
the flat lowlands of a delta plain are the prototype of an
environment, where even small sea-level oscillations can
affect the depositional processes of large areas.
- Delta-lobe switching, constructional and destructional phases, and response to eustatic sealevel changes create complex lateral and vertical facies associations, including cyclic sequences.
- Buried peats, growth faults and depobelts in
conjunction with prodelta deposits provide targets for coal and hydrocarbon exploration.