7.4 Marine Deltas, Deep-Sea Fans, and Oceanic Sediments
325
7.3.4 Summary (Coastal and Shallow-Marine Siliciclastic Systems)
The sediments most affected by relative sealevel changes are those of the coastal and
shallow-marine zones.
High-frequency sea-level changes can cause
rapid progradation (forced regression) or retreat
of the coastline, including the drowning of
barrier-lagoon systems.
Truncated, sharp-based shoreface sequences indicate a lowered storm-wave base.
Limited sediment supply leads to non-accretionary regressive and transgressive systems and
to widely extended lags in epicontinental seas or
on the inner and middle shelf. The lags merge
7.4 Marine Deltas, Deep-Sea Fans,
and Oceanic Sediments
7.4.1 Response ofDeltas to Sea-Level Changes
Cyclic phenomena in marine deltas originate from both
autogenetic and allogenetic processes. Irregular switching of delta lobes, related to the degree of sediment accumulation and the geometry ofthe delta complex, may
lead to altemations between sediments ofthe delta plain
and marine incursions (cf. Sect. 3.5). Interdistributary
areas cut off from sediment supply subside below sea
level and are flooded either by fresh water or the sea
(cf. Fig. 3.36). When deltaic progradationreturns to this
area, marsh, lagoonal, and shallow-marine layers are
again covered by sediments of the fluvial delta plain. In
this way, a purely autocyclic sequence may evolve with
altemating fluvial, lagoonal, lacustrine, and marine sediments.
For example, Holoeene (?) autoeycles in the Mississippi delta
plain span a time period of approximately 1000 to 1500 years
(Tye and Kosters 1986). However, as in the ease oflow-lying
river plains, these autoeyclie proeesses ean be overprinted and
modified by global (allogenetie) sea-Ievel oseillations.
One of the means for diseriminating between allogenetie
and autogenetie proeesses is stratigraphie eorrelation. While
global and regional phenomena ean be traeed over long distanees and from basin to basin, beds and sedimentary eycles
related to autoeyclie proeesses pineh out in a relatively short
distanee.
To study the response of deltaic sediments to sea-level
changes, the general concepts developed for siliciclastics-dominated ramp margins and shelf-slope settings can be used. F urthermore, deltas prograding to the
edge of deep oceanic basins initiate a composite
depositional system consisting ofthe delta, the prodelta,
and an associated deep-sea fan. The relationship between these different depositional areas, however, is
landward due to decreasing subsidence.
The number of cycles and the degree of cycle
symmetry (or asymmetry) often varies from
marginal to more central locations within a basin.
High sand supply can create extensive sand
sheets and sometimes isolated sand bodies
which are later buried under mud.
Moderate sediment supply, differential subsidence, and sea-level fall below the shelf edge
generate the classical model of sequence stratigraphy in shelf-slope-deep basin settings.
complex because each of these systems is variable
(Sects. 3.5 and 5.4.2). The effects of relative sea-level
changes on delta plains and delta fronts are indicated in
Section 11.5.2 and illustrated in Figure 3.36. Here, only
some further general points can be mentioned.
The principal interplay between sediment supply,
frequency and amplitude of sea-level changes is demonstrated in Figure 7.19. In all cases, the delta front
pro grades with relative sea-level fall. The rate of
prograding slows with increasing water depth and diminishing terrigenous sediment supply. During relative
sea-level rise, the delta front normally steps back, but
even in this phase the delta may pro grade when the amplitude of sea-level change is limited (slow rise) and the
sediment supply is high (Fig. 7.19; cf. Fig. 11.24d).
The 2 D-models are based on the assumption that sediment
supply from distant sourees is eonstant, but that some additional sediment is made available from loeal sourees byvalley
eutting during falling sea level. The deltas prograde into shallow seas; subsidenee is negleeted. The models illustrate the
inereasing rates of prograding during 10wstand whieh is
eaused by deereasing aeeommodation spaee while the mass
of sediment delivered per time unit remains eonstant or even
inereases.
As a result of the Quaternary high-amplitude, high-frequeney sea-Ievel ehanges, the young history of modern deltas
generallyreeords backstepping during the rapid Ho10cene sealevel rise. This applies, for example, to the deltas ofthe Mediterranean Sea, such as the Nile delta (Coutellier and Stanley
1987), the Rhone delta (Tesson et al. 1990; Gensous et al.
1993), the deltas ofthe Po and Tiber (cf. Sect. 11.5.2 andFig.
11.25). Backstepping also occurred at most ofthe major river
deltas ofthe large oeean basins such as the lndus river (von
Rad and Tahir 1997, see also below). An exeeption to this rule
is the Fraser river delta (cf. Fig. 11.26) because here Holocene
relative sea-level rise was strong1y reduced by coastal uplift.
The Ho10cene evolution of the Ebro delta in SE Spain
exhibits an overall transgressive trend (up to about 7 ka B.P.)
followed by a regressive phase (since 7 ka B.P.). These phases
are superimposed by 6th order sequences (cycle period of
about 1.5 ka) generating prograding and backstepping faeies
325
7.3.4 Summary (Coastal and Shallow-Marine Siliciclastic Systems)
The sediments most affected by relative sealevel changes are those of the coastal and
shallow-marine zones.
High-frequency sea-level changes can cause
rapid progradation (forced regression) or retreat
of the coastline, including the drowning of
barrier-lagoon systems.
Truncated, sharp-based shoreface sequences indicate a lowered storm-wave base.
Limited sediment supply leads to non-accretionary regressive and transgressive systems and
to widely extended lags in epicontinental seas or
on the inner and middle shelf. The lags merge
7.4 Marine Deltas, Deep-Sea Fans,
and Oceanic Sediments
7.4.1 Response ofDeltas to Sea-Level Changes
Cyclic phenomena in marine deltas originate from both
autogenetic and allogenetic processes. Irregular switching of delta lobes, related to the degree of sediment accumulation and the geometry ofthe delta complex, may
lead to altemations between sediments ofthe delta plain
and marine incursions (cf. Sect. 3.5). Interdistributary
areas cut off from sediment supply subside below sea
level and are flooded either by fresh water or the sea
(cf. Fig. 3.36). When deltaic progradationreturns to this
area, marsh, lagoonal, and shallow-marine layers are
again covered by sediments of the fluvial delta plain. In
this way, a purely autocyclic sequence may evolve with
altemating fluvial, lagoonal, lacustrine, and marine sediments.
For example, Holoeene (?) autoeycles in the Mississippi delta
plain span a time period of approximately 1000 to 1500 years
(Tye and Kosters 1986). However, as in the ease oflow-lying
river plains, these autoeyclie proeesses ean be overprinted and
modified by global (allogenetie) sea-Ievel oseillations.
One of the means for diseriminating between allogenetie
and autogenetie proeesses is stratigraphie eorrelation. While
global and regional phenomena ean be traeed over long distanees and from basin to basin, beds and sedimentary eycles
related to autoeyclie proeesses pineh out in a relatively short
distanee.
To study the response of deltaic sediments to sea-level
changes, the general concepts developed for siliciclastics-dominated ramp margins and shelf-slope settings can be used. F urthermore, deltas prograding to the
edge of deep oceanic basins initiate a composite
depositional system consisting ofthe delta, the prodelta,
and an associated deep-sea fan. The relationship between these different depositional areas, however, is
landward due to decreasing subsidence.
The number of cycles and the degree of cycle
symmetry (or asymmetry) often varies from
marginal to more central locations within a basin.
High sand supply can create extensive sand
sheets and sometimes isolated sand bodies
which are later buried under mud.
Moderate sediment supply, differential subsidence, and sea-level fall below the shelf edge
generate the classical model of sequence stratigraphy in shelf-slope-deep basin settings.
complex because each of these systems is variable
(Sects. 3.5 and 5.4.2). The effects of relative sea-level
changes on delta plains and delta fronts are indicated in
Section 11.5.2 and illustrated in Figure 3.36. Here, only
some further general points can be mentioned.
The principal interplay between sediment supply,
frequency and amplitude of sea-level changes is demonstrated in Figure 7.19. In all cases, the delta front
pro grades with relative sea-level fall. The rate of
prograding slows with increasing water depth and diminishing terrigenous sediment supply. During relative
sea-level rise, the delta front normally steps back, but
even in this phase the delta may pro grade when the amplitude of sea-level change is limited (slow rise) and the
sediment supply is high (Fig. 7.19; cf. Fig. 11.24d).
The 2 D-models are based on the assumption that sediment
supply from distant sourees is eonstant, but that some additional sediment is made available from loeal sourees byvalley
eutting during falling sea level. The deltas prograde into shallow seas; subsidenee is negleeted. The models illustrate the
inereasing rates of prograding during 10wstand whieh is
eaused by deereasing aeeommodation spaee while the mass
of sediment delivered per time unit remains eonstant or even
inereases.
As a result of the Quaternary high-amplitude, high-frequeney sea-Ievel ehanges, the young history of modern deltas
generallyreeords backstepping during the rapid Ho10cene sealevel rise. This applies, for example, to the deltas ofthe Mediterranean Sea, such as the Nile delta (Coutellier and Stanley
1987), the Rhone delta (Tesson et al. 1990; Gensous et al.
1993), the deltas ofthe Po and Tiber (cf. Sect. 11.5.2 andFig.
11.25). Backstepping also occurred at most ofthe major river
deltas ofthe large oeean basins such as the lndus river (von
Rad and Tahir 1997, see also below). An exeeption to this rule
is the Fraser river delta (cf. Fig. 11.26) because here Holocene
relative sea-level rise was strong1y reduced by coastal uplift.
The Ho10cene evolution of the Ebro delta in SE Spain
exhibits an overall transgressive trend (up to about 7 ka B.P.)
followed by a regressive phase (since 7 ka B.P.). These phases
are superimposed by 6th order sequences (cycle period of
about 1.5 ka) generating prograding and backstepping faeies