316
Chapter 7 Sequences, Minor Cycles, and Event Stratigraphy
Channel) and the Gulf of Lion (Mediterranean, off southem
France; Beme et al. 1998). The sand bodies are ten to several
tens ofkm wide and 20 to 50 m high. They fonned during the
last sea-levellowstand as shelf-perched lowstand wedges (cf.
Fig. 7.20) which were partlyreworked bythe following transgression. Even in the high-energy tide- and wave-dominated
environment ofthe Celtic Sea the major part ofthe lowstand
sand bodies is preserved due to the rapid Holocene sea-level
nse.
Coasts along active continental margins are frequently
affected by uplift generally causing the shoreline to
migrate basinward. This long-term trend may be superimposed by sea-level changes. During times in which
the rates of sea-level rise and uplift are approximately
equal (Fig. 7.l3d), the shoreline tends to be stable, favoring the formation of cliffs. During sea-level fall, the
shoreline pro grades rapidly, possiblyfurther accelerated
by increased sediment supply from valley incision in the
hinterland.
7.3.2 Ramp Margins and Epicontinental Seas
Effects ofVariable Sediment Influx and Subsidence
The influx ofterrigenous sediment and the rates of differential subsidence have a profound impact on the facies and sediment buildup on a ramp-type basin margin.
This is demonstrated by three schematic time-space
diagrams (Fig. 7.14a), in wh ich both sediment supply
rich in sand and subsidence increase from the left- to
the right-hand side. The sand-poor, low-subsidence
model displays reworked horizons (regressive and
transgressive lags) between shales and limited sand accumulation during the lowstand phase. In the sand-rich,
high-subsidence model, a thick sand belt migrates
basinward or landward during relative SLF or SLR,
respectively, and largely fills the accommodation space
provided by subsidence. Erosionallags only occur on
the inner ramp. Between these two end members many
transitional depositional systems are feasible.
Ramp Margin, High Sand Supply
The models ofFigure 7.14b and c describe third order
or higher order depositional sequences developing on
Fig. 7.13. a Different modes of shoreline retreat controlled by the rate of sea-level rise and sediment supply, SS. Limited SS and rapid rise cause drowning of
older barrier-lagoon systems. (After Einseie 1993,
based on Elliott 1986, modified). b Prograding and
retreating sand-rich coastlines controlled by the rate
of relative sea-level change. Note erosional surfaces,
produced during both regression and transgression,
as weil as the formation of isolated shelf sand bodies.
(Eins eie 1993, modified from Plint 1988). eNormal
a ramp margin or in an epicontinental sea when sand
supply is relatively high. Consequently, the systems
tracts or parasequences deposited in shallow water are
rich in sand. If subsidence creates more space than can
be filled by the incoming sediment flux, the total sequence displays a backstepping trend with time (Fig.
7.14b), whereas a surplus in sediment supply will cause
subsequent parasequences to pro grade over the older
ones (Fig. 7.14c). In the latter example, swamps and
coal seams may form on the aggrading coastal plain
during the late transgressive phase, when the gradients
of incoming rivers are reduced and the rising groundwater table more or less reaches the land surface. The coal
is buried under thin transgressive muds ± shallow-marine carbonate layers.
Non-Uniform SL-Changes, Low Sediment Supply
The model of Figure 7.15 describes the response of
sediments to a number of non-uniform sea-level
changes in a cross-section through the broad marginal
zone of a slowly differentially subsiding basin, such as
an epicontinental sea. The basin receives mainly fine
grained terrigenous material which enables long-term
sediment accumulation (below the storm-wave base of
low sea level) to approximately keep pace with subsidence. In the coastal and foreshore zones, maximum
SLF is greater than SUB. Due to lack of sand, seaward
prograding coastal sand bodies playa minor role. Both
the transgressive and regressive phase of sea-level
change are recorded by lag sediments (Fig. 7.16a and
b, cf. Sect. 7.9).
Their landward and seaward extent is controlled by the varying amp!itudes ofthe sea-level curve. Ifflexural subsidence
landward ofthe hinge !ine and continental deposition can be
neglected, the transgressive lag tends to rest directly on the
previous regressive lag or on older bedrock. The lag horizon
may contain a mixed fauna of differing age as weil as re!ics
of older beds or concretions. The duration ofthe stratigraphic
hiatus between the two types oflags increases landward (Fig.
7.15b; cf. Fig. 7. lOb). Here, a considerable part ofthe transgressive sediments can be removed bymechanical and chemical denudation during the subsequent regressive phase which,
in the case of 3rd order cycles, may last about 1 Ma. Another
part ofthe transgressive sequence is eroded by the lowering
stonn-wave base (Fig. 7.l5a; cf. Fig. 7.7b).
and sharp-based foreshore section generated by wave
erosion during relative sea-level fall (see b2). d Stepwise basinward shoreline migration due to relative
sea-level change in regions of coastal uplift. (From
Einseie 1996). e Parasequences, PS, produced by
regressive barrier-lagoon systems and transgressive
shelf muds during high-frequency (Pleistocene) sealevel oscillations along a sand-rich coast (Atlantic
coast of Brazil. (After Dominguez et al. 1992, modified)
Chapter 7 Sequences, Minor Cycles, and Event Stratigraphy
Channel) and the Gulf of Lion (Mediterranean, off southem
France; Beme et al. 1998). The sand bodies are ten to several
tens ofkm wide and 20 to 50 m high. They fonned during the
last sea-levellowstand as shelf-perched lowstand wedges (cf.
Fig. 7.20) which were partlyreworked bythe following transgression. Even in the high-energy tide- and wave-dominated
environment ofthe Celtic Sea the major part ofthe lowstand
sand bodies is preserved due to the rapid Holocene sea-level
nse.
Coasts along active continental margins are frequently
affected by uplift generally causing the shoreline to
migrate basinward. This long-term trend may be superimposed by sea-level changes. During times in which
the rates of sea-level rise and uplift are approximately
equal (Fig. 7.l3d), the shoreline tends to be stable, favoring the formation of cliffs. During sea-level fall, the
shoreline pro grades rapidly, possiblyfurther accelerated
by increased sediment supply from valley incision in the
hinterland.
7.3.2 Ramp Margins and Epicontinental Seas
Effects ofVariable Sediment Influx and Subsidence
The influx ofterrigenous sediment and the rates of differential subsidence have a profound impact on the facies and sediment buildup on a ramp-type basin margin.
This is demonstrated by three schematic time-space
diagrams (Fig. 7.14a), in wh ich both sediment supply
rich in sand and subsidence increase from the left- to
the right-hand side. The sand-poor, low-subsidence
model displays reworked horizons (regressive and
transgressive lags) between shales and limited sand accumulation during the lowstand phase. In the sand-rich,
high-subsidence model, a thick sand belt migrates
basinward or landward during relative SLF or SLR,
respectively, and largely fills the accommodation space
provided by subsidence. Erosionallags only occur on
the inner ramp. Between these two end members many
transitional depositional systems are feasible.
Ramp Margin, High Sand Supply
The models ofFigure 7.14b and c describe third order
or higher order depositional sequences developing on
Fig. 7.13. a Different modes of shoreline retreat controlled by the rate of sea-level rise and sediment supply, SS. Limited SS and rapid rise cause drowning of
older barrier-lagoon systems. (After Einseie 1993,
based on Elliott 1986, modified). b Prograding and
retreating sand-rich coastlines controlled by the rate
of relative sea-level change. Note erosional surfaces,
produced during both regression and transgression,
as weil as the formation of isolated shelf sand bodies.
(Eins eie 1993, modified from Plint 1988). eNormal
a ramp margin or in an epicontinental sea when sand
supply is relatively high. Consequently, the systems
tracts or parasequences deposited in shallow water are
rich in sand. If subsidence creates more space than can
be filled by the incoming sediment flux, the total sequence displays a backstepping trend with time (Fig.
7.14b), whereas a surplus in sediment supply will cause
subsequent parasequences to pro grade over the older
ones (Fig. 7.14c). In the latter example, swamps and
coal seams may form on the aggrading coastal plain
during the late transgressive phase, when the gradients
of incoming rivers are reduced and the rising groundwater table more or less reaches the land surface. The coal
is buried under thin transgressive muds ± shallow-marine carbonate layers.
Non-Uniform SL-Changes, Low Sediment Supply
The model of Figure 7.15 describes the response of
sediments to a number of non-uniform sea-level
changes in a cross-section through the broad marginal
zone of a slowly differentially subsiding basin, such as
an epicontinental sea. The basin receives mainly fine
grained terrigenous material which enables long-term
sediment accumulation (below the storm-wave base of
low sea level) to approximately keep pace with subsidence. In the coastal and foreshore zones, maximum
SLF is greater than SUB. Due to lack of sand, seaward
prograding coastal sand bodies playa minor role. Both
the transgressive and regressive phase of sea-level
change are recorded by lag sediments (Fig. 7.16a and
b, cf. Sect. 7.9).
Their landward and seaward extent is controlled by the varying amp!itudes ofthe sea-level curve. Ifflexural subsidence
landward ofthe hinge !ine and continental deposition can be
neglected, the transgressive lag tends to rest directly on the
previous regressive lag or on older bedrock. The lag horizon
may contain a mixed fauna of differing age as weil as re!ics
of older beds or concretions. The duration ofthe stratigraphic
hiatus between the two types oflags increases landward (Fig.
7.15b; cf. Fig. 7. lOb). Here, a considerable part ofthe transgressive sediments can be removed bymechanical and chemical denudation during the subsequent regressive phase which,
in the case of 3rd order cycles, may last about 1 Ma. Another
part ofthe transgressive sequence is eroded by the lowering
stonn-wave base (Fig. 7.l5a; cf. Fig. 7.7b).
and sharp-based foreshore section generated by wave
erosion during relative sea-level fall (see b2). d Stepwise basinward shoreline migration due to relative
sea-level change in regions of coastal uplift. (From
Einseie 1996). e Parasequences, PS, produced by
regressive barrier-lagoon systems and transgressive
shelf muds during high-frequency (Pleistocene) sealevel oscillations along a sand-rich coast (Atlantic
coast of Brazil. (After Dominguez et al. 1992, modified)