7.2 Basic Concepts
the following models it is assumed that basinward from
a hinge line (separating areas of subsidence and uplift)
subsidence increases basinward. The amount of subsidence rnay increase linearly with the distance from the
hinge line (rotational subsidence, Fig. 7.1 Oa) or it may
grow in a less regular way.
The different components of total subsidence are neglected
here as is sediment compaction. Flexure of the underlying
crost, resulting from the increasing sediment load in the basin,
can also cause subsidence landward ofthe hinge line (cf. Sect.
8.4).
On ramp margins differential subsidence during relative
SLF (half-cyc1e) may lead to the following processes
(Fig.7.l0a):
- Landward from the equilibrium point (SLF=SUB),
accommodation space (ACC) is lost, which leads to
emergence and subaerial erosion, inc1uding the incision
of valleys. The emerged zone and its morphological
features will adjust to the conditions of weathering and
subaerial sediment transport. The shoreline will move
basinward, but its precise position is controlled by sediment supply and the mode of redistribution of the
eroded and incoming new sediment. The maximum
thickness of the newly accumulating sediment can be
expected seaward from the equilibrium point where
ACC is gained even during SLF.
- When SL again rises, the maximum thickness of the
new increment of sediment shifts landward. However,
this sediment mass usually does not contain eroded
olderrnaterial. Generally, the sedimentmasses available
during base-level rise or fall should be taken into account in all considerations about sequence stratigraphy.
In other words, the principle of mass conservation
should not be neglected (e.g. Cross 1997).
According to this concept, the sequence cycle consists of only
two facies tracts: the progradational base-level fall half-cycle
and the aggradational base-level rise half-cycle. The genetic
sequence (full cycle) contains the total amount of sediment
accumulated in alliinked depositional environments.
- Basinward increasing rates of subsidence also affect
the nature of the sequence boundaries as displayed in
the time-space diagram (Wheeler diagram) of Fig.
7. lOb. Type 1 boundaries (SB 1, erosional unconformities) pass into Type 2 boundaries (SB2) which do not
show any hiatus. Isochrones (time lines) in the cross
section merge landward and combine to erosional unconformities (cf. Fig. 7.15d; see also Ross 1989). The
basinward extent of SB 1 increases with the amplitude
of sea-level change.
- For a given amount of sediment supply, SS, or potential sediment buildup SEDp>SUB, the hiatus of SB1
starts earlier and ends later c10se to the hinge line than
farther basinward where it finally ceases and is replaced
by a correlative conformity (Fig. 7.l0b). The duration
of the hiatus for a certain location within the basin in311
creases with increasing sediment supply and vice versa.
- Sequence boundaries and other specific surfaces generally do not coincide with the lowest or highest points
ofthe eustatic sea-1evel curve, but may migrate up and
down in time according to their position near to or far
away from the edge of the basin.
- The maximum flooding surface (MFS) signifies a
situation in which the basin margin reaches its maximum water depth.
The position ofMFS on the time scale also depends on SS or
SEDp as weil as on the subsidence rate. With SEDp>SUB,
maximum flooding occurs prior to the peak ofthe eustatic sealevel curve. In a strict sense, neither the maximum flooding
surface and the condensed section, nor the boundaries ofthe
systems tracts (also see below) can be assumed to be synchronous (Wehr 1993). This is so even in the same basin, when
sediment supply and/or subsidence differ from one part ofthe
basin to the other.
The basin cross section of Fig. 7.l0c shows the
basinward increasing sediment thicknesses of model (a)
and allows surplus sediment to bui1d prograding sediment bodies (± sandy longshore bars or seaward dipping sediment wedges). These correspond to the
lowstand systems tract (LST) of the "c1assical" shelfbreak model of the EXXON group. SB 1 ends where
SUB is always greater than SLF.
With the onset of relative sea-level rise, sediments
begin to aggrade on top of SB2, while outbuilding continues at a slowing rate. Then lack of sediment with
respect to increasing ACC causes backstepping and
coastal onlap (transgressive systems tract, TST) until
the maximum flooding surface, MFS, is established.
The following highstand deposits (HST) again
pro grade. This model also largely explains the shelfbreak model developed for passive margin settings (see
below).
The general principles discussed above can be applied to 3rd order sequences as well as to higher frequency sequences (4th and 5th order). However, the
superposition oftwo or several frequencies in conjunction with differential subsidence and other factors create a variety of phenomena which cannot be described
by one or only a few facies models (for many regional
examples see e.g. Miall1997). The few idealized models discussed below demonstrate some of these comp1exities. Basin modelling with the aid of computers
provides a more sophisticated approach to a better understanding of these complex scenarios (e.g. Lawrence
1993; Levell and Leu 1993; and others).
In this context the results of two computer models
are ofinterest (Fig. 7.11; Levell and Leu 1993). They
show that the overall sediment architecture of a shelfslope setting subjected to cyc1ic variation in sediment
supply (Fig. 7.11 b, stable sea level) is similar to that
produced by sea-level changes (Fig. 7.11a, constant
sediment supply). However, a principal difference
arises between the two models in the occurrence and
nature of sequence boundaries and coastal onlap.
the following models it is assumed that basinward from
a hinge line (separating areas of subsidence and uplift)
subsidence increases basinward. The amount of subsidence rnay increase linearly with the distance from the
hinge line (rotational subsidence, Fig. 7.1 Oa) or it may
grow in a less regular way.
The different components of total subsidence are neglected
here as is sediment compaction. Flexure of the underlying
crost, resulting from the increasing sediment load in the basin,
can also cause subsidence landward ofthe hinge line (cf. Sect.
8.4).
On ramp margins differential subsidence during relative
SLF (half-cyc1e) may lead to the following processes
(Fig.7.l0a):
- Landward from the equilibrium point (SLF=SUB),
accommodation space (ACC) is lost, which leads to
emergence and subaerial erosion, inc1uding the incision
of valleys. The emerged zone and its morphological
features will adjust to the conditions of weathering and
subaerial sediment transport. The shoreline will move
basinward, but its precise position is controlled by sediment supply and the mode of redistribution of the
eroded and incoming new sediment. The maximum
thickness of the newly accumulating sediment can be
expected seaward from the equilibrium point where
ACC is gained even during SLF.
- When SL again rises, the maximum thickness of the
new increment of sediment shifts landward. However,
this sediment mass usually does not contain eroded
olderrnaterial. Generally, the sedimentmasses available
during base-level rise or fall should be taken into account in all considerations about sequence stratigraphy.
In other words, the principle of mass conservation
should not be neglected (e.g. Cross 1997).
According to this concept, the sequence cycle consists of only
two facies tracts: the progradational base-level fall half-cycle
and the aggradational base-level rise half-cycle. The genetic
sequence (full cycle) contains the total amount of sediment
accumulated in alliinked depositional environments.
- Basinward increasing rates of subsidence also affect
the nature of the sequence boundaries as displayed in
the time-space diagram (Wheeler diagram) of Fig.
7. lOb. Type 1 boundaries (SB 1, erosional unconformities) pass into Type 2 boundaries (SB2) which do not
show any hiatus. Isochrones (time lines) in the cross
section merge landward and combine to erosional unconformities (cf. Fig. 7.15d; see also Ross 1989). The
basinward extent of SB 1 increases with the amplitude
of sea-level change.
- For a given amount of sediment supply, SS, or potential sediment buildup SEDp>SUB, the hiatus of SB1
starts earlier and ends later c10se to the hinge line than
farther basinward where it finally ceases and is replaced
by a correlative conformity (Fig. 7.l0b). The duration
of the hiatus for a certain location within the basin in311
creases with increasing sediment supply and vice versa.
- Sequence boundaries and other specific surfaces generally do not coincide with the lowest or highest points
ofthe eustatic sea-1evel curve, but may migrate up and
down in time according to their position near to or far
away from the edge of the basin.
- The maximum flooding surface (MFS) signifies a
situation in which the basin margin reaches its maximum water depth.
The position ofMFS on the time scale also depends on SS or
SEDp as weil as on the subsidence rate. With SEDp>SUB,
maximum flooding occurs prior to the peak ofthe eustatic sealevel curve. In a strict sense, neither the maximum flooding
surface and the condensed section, nor the boundaries ofthe
systems tracts (also see below) can be assumed to be synchronous (Wehr 1993). This is so even in the same basin, when
sediment supply and/or subsidence differ from one part ofthe
basin to the other.
The basin cross section of Fig. 7.l0c shows the
basinward increasing sediment thicknesses of model (a)
and allows surplus sediment to bui1d prograding sediment bodies (± sandy longshore bars or seaward dipping sediment wedges). These correspond to the
lowstand systems tract (LST) of the "c1assical" shelfbreak model of the EXXON group. SB 1 ends where
SUB is always greater than SLF.
With the onset of relative sea-level rise, sediments
begin to aggrade on top of SB2, while outbuilding continues at a slowing rate. Then lack of sediment with
respect to increasing ACC causes backstepping and
coastal onlap (transgressive systems tract, TST) until
the maximum flooding surface, MFS, is established.
The following highstand deposits (HST) again
pro grade. This model also largely explains the shelfbreak model developed for passive margin settings (see
below).
The general principles discussed above can be applied to 3rd order sequences as well as to higher frequency sequences (4th and 5th order). However, the
superposition oftwo or several frequencies in conjunction with differential subsidence and other factors create a variety of phenomena which cannot be described
by one or only a few facies models (for many regional
examples see e.g. Miall1997). The few idealized models discussed below demonstrate some of these comp1exities. Basin modelling with the aid of computers
provides a more sophisticated approach to a better understanding of these complex scenarios (e.g. Lawrence
1993; Levell and Leu 1993; and others).
In this context the results of two computer models
are ofinterest (Fig. 7.11; Levell and Leu 1993). They
show that the overall sediment architecture of a shelfslope setting subjected to cyc1ic variation in sediment
supply (Fig. 7.11 b, stable sea level) is similar to that
produced by sea-level changes (Fig. 7.11a, constant
sediment supply). However, a principal difference
arises between the two models in the occurrence and
nature of sequence boundaries and coastal onlap.