7.3 Coastal and Shallow-Marine Siliciclastic Systems
The time steps ofthe simulation are 0.5 Ma. At the beginning
the basin is shallow and its shoreline lies at the hinge line. For
both models rotational differential subsidence around the
hinge li ne and isostatic adjustment of the underlying crnst to
the increasing sediment load is assumed. Basinward decreasing sediment transport creates a depositional shelf break.
Model (a) produces prograding and backstepping sediment
bodies as weil as other characteristic features of the seismic
stratigraphic concept for passive continental margins (Sect.
313
7.3.3; cf. Fig. 7.3f and g). Model (b) displays a maximum
flooding surface (MFS), as (a), but does notdevelop the same
type of sequence boundaries. Prograding during phases of
high sediment supply is restricted to the outerpart ofthe cross
section, whereas the central part mainly shows vertical
aggradation. Distinct coastal onlap only occurs landward of
the hinge line.
7.2.5 Summary (Sequence Stratigraphy, General Concepts)
Modem seismic stratigraphy and sequence stratigraphy evolved from the study of Atlantic-type
passive margins. This concept is now applied
with a number of modifications to all types of
basin setting.
- Relative sea- or base-level changes affect the
sediment-accommodation space for the buildup
of sediment in coastal and shallow-sea areas as
weIl as on land and cause the depocenter to shift
landward or seaward. This leads to prograding
or backstepping facies associations (systems
tracts or facies tracts).
- A systems tract normally consists of sediments
of different dcpositional environments (marine
and continental). Systems tracts can often be
subdivided into parasequences which show distinct stacking patterns.
7.3 Sequence Stratigraphy ofCoastal and
Shallow-Marine Siliciclastic Systems
7.3.1 Coastline Migration
Most ofthe following model scenarios reflect field observations. Differential subsidence is taken into account
for the shelf and shelf-break-slope settings. Major marine deltas are discussed separately (Sect. 7.4).
In terms ofterrigenous sediment supply one can distinguish two end members: (1) scenarios with abundant
and (2) very limited supply. As a result, the coastline
and foreshore zone may be either accretionary or nonaccretionary (Fig. 7.12; Helland-Hansen and Martinsen
1997). Non-accretionary settings are characterized by
pronounced regressive and transgressive lags.
Accretionary settings build prograding sediment bodies
during stable and slowly rising sea levels as well as
during relative sea-Ievel fall. Backstepping sediment
bodies result from faster sea-Ievel rise. Composite patterns evolve when accretionary normal or forced regression is followed by non-accretionarytransgression (Fig.
7 .12f and g). Settings controlled mainly by episodic
relative sea-level rise (or subsidence) and sufficient
sediment supply tend to generate stepwise prograding
The interplay of sea-Ievel change, supply and
aggradation of sediment, and (often differential) subsidence generates a variety of phenomena which cannot always be interpreted unambiguously. Additional complications result from
the superposition of sea-Ievel fluctuations of
differing frequency and amplitude and variations in sediment supply.
- The SB I sequence boundary occurs in areas
where relative sea-Ievel fall is greater than subsidence (+ syngenetic sediment buildup). This
unconformity is not synchronous and normally
not a basin-wide surface. The correlative SB2 is
mostly difficult to identify. Maximum flooding
surfaces and condensed sections are frequently
useful as ± synchronous marker horizons.
coastal sand bodies. Settings undergoing fast subsidence and receiving limited amounts of sand are characterized by backstepping cycles of coastal sands.
The landward or seaward migration of coastal zones
depends on the interplay between sea-Ievel change, sediment ag gradation, and subsidence (or uplift). In the
case of slow subsidence and limited sediment supply,
for example, even a slow rate of eustatic sea-level fall
can reduce the sediment accommodation space and thus
lead to a substantial basinward shift ofthe coastal facies
association.
Assuming a ramp basin slope of 2%0 (2 m per 1 km) and neglecting subsidence and sediment redistribution, a 3rd order
sea-Ievel fall (half-period 1 Ma, amplitude 100 m) can force
the shoreline to migrate basinward at an average rate ofO.05
mla. However, a 5th order sea-Ievel cyc1e similar to the late
Pleistocene sea-Ievel oscillations (half-period 10 ka, amplitude 100 m) will shift the shoreline at a rate of 5 mla (e.g.
EinseIe 1996).
Likewise, rapid sea-Ievel rise can generate fast shoreline retreat and drown older coastal barriers, whereas
slowtransgressions tend to rework pre-existing barriers
and shift them landward (Fig. 7.13a). Thus they establish a new kind of equilibrium profile on the shoreface.
The time steps ofthe simulation are 0.5 Ma. At the beginning
the basin is shallow and its shoreline lies at the hinge line. For
both models rotational differential subsidence around the
hinge li ne and isostatic adjustment of the underlying crnst to
the increasing sediment load is assumed. Basinward decreasing sediment transport creates a depositional shelf break.
Model (a) produces prograding and backstepping sediment
bodies as weil as other characteristic features of the seismic
stratigraphic concept for passive continental margins (Sect.
313
7.3.3; cf. Fig. 7.3f and g). Model (b) displays a maximum
flooding surface (MFS), as (a), but does notdevelop the same
type of sequence boundaries. Prograding during phases of
high sediment supply is restricted to the outerpart ofthe cross
section, whereas the central part mainly shows vertical
aggradation. Distinct coastal onlap only occurs landward of
the hinge line.
7.2.5 Summary (Sequence Stratigraphy, General Concepts)
Modem seismic stratigraphy and sequence stratigraphy evolved from the study of Atlantic-type
passive margins. This concept is now applied
with a number of modifications to all types of
basin setting.
- Relative sea- or base-level changes affect the
sediment-accommodation space for the buildup
of sediment in coastal and shallow-sea areas as
weIl as on land and cause the depocenter to shift
landward or seaward. This leads to prograding
or backstepping facies associations (systems
tracts or facies tracts).
- A systems tract normally consists of sediments
of different dcpositional environments (marine
and continental). Systems tracts can often be
subdivided into parasequences which show distinct stacking patterns.
7.3 Sequence Stratigraphy ofCoastal and
Shallow-Marine Siliciclastic Systems
7.3.1 Coastline Migration
Most ofthe following model scenarios reflect field observations. Differential subsidence is taken into account
for the shelf and shelf-break-slope settings. Major marine deltas are discussed separately (Sect. 7.4).
In terms ofterrigenous sediment supply one can distinguish two end members: (1) scenarios with abundant
and (2) very limited supply. As a result, the coastline
and foreshore zone may be either accretionary or nonaccretionary (Fig. 7.12; Helland-Hansen and Martinsen
1997). Non-accretionary settings are characterized by
pronounced regressive and transgressive lags.
Accretionary settings build prograding sediment bodies
during stable and slowly rising sea levels as well as
during relative sea-Ievel fall. Backstepping sediment
bodies result from faster sea-Ievel rise. Composite patterns evolve when accretionary normal or forced regression is followed by non-accretionarytransgression (Fig.
7 .12f and g). Settings controlled mainly by episodic
relative sea-level rise (or subsidence) and sufficient
sediment supply tend to generate stepwise prograding
The interplay of sea-Ievel change, supply and
aggradation of sediment, and (often differential) subsidence generates a variety of phenomena which cannot always be interpreted unambiguously. Additional complications result from
the superposition of sea-Ievel fluctuations of
differing frequency and amplitude and variations in sediment supply.
- The SB I sequence boundary occurs in areas
where relative sea-Ievel fall is greater than subsidence (+ syngenetic sediment buildup). This
unconformity is not synchronous and normally
not a basin-wide surface. The correlative SB2 is
mostly difficult to identify. Maximum flooding
surfaces and condensed sections are frequently
useful as ± synchronous marker horizons.
coastal sand bodies. Settings undergoing fast subsidence and receiving limited amounts of sand are characterized by backstepping cycles of coastal sands.
The landward or seaward migration of coastal zones
depends on the interplay between sea-Ievel change, sediment ag gradation, and subsidence (or uplift). In the
case of slow subsidence and limited sediment supply,
for example, even a slow rate of eustatic sea-level fall
can reduce the sediment accommodation space and thus
lead to a substantial basinward shift ofthe coastal facies
association.
Assuming a ramp basin slope of 2%0 (2 m per 1 km) and neglecting subsidence and sediment redistribution, a 3rd order
sea-Ievel fall (half-period 1 Ma, amplitude 100 m) can force
the shoreline to migrate basinward at an average rate ofO.05
mla. However, a 5th order sea-Ievel cyc1e similar to the late
Pleistocene sea-Ievel oscillations (half-period 10 ka, amplitude 100 m) will shift the shoreline at a rate of 5 mla (e.g.
EinseIe 1996).
Likewise, rapid sea-Ievel rise can generate fast shoreline retreat and drown older coastal barriers, whereas
slowtransgressions tend to rework pre-existing barriers
and shift them landward (Fig. 7.13a). Thus they establish a new kind of equilibrium profile on the shoreface.