7.3 Coastal and Shallow-Marine Siliciclastic Systems
The chronostratigraphic block diagram in Figure 7.17a
shows the development oftwo sequences. Similar to the
models in Figure 7.15, it is assumed that the subsidence
rate increases from a hinge line toward the basin center.
The influx of terrigenous material is moderate and the
sediments are predominantly siliciclastic. During the
transgressive and early highstand phase, most of the
incoming sediment is trapped in shallow water and on
the shelf, while little material is transported into deeper
water. During falling sea level and lowstand, the locus
of major sediment accumulation is the slope and continental rise, where relatively high sedimentation rates are
reached. The systems tracts of a third order sequence
can be characterized as follows:
- The lowstand systems tract (LS T) develops during the
phase of rapid eustatic sea-level fall up to the early part
ofthe rise (Fig. 7.l7a). Its specific characteristics on
passive margins are a basin floor fan and several slope
fans overlain by a lowstand wedge. The latter results
from continued prograding and some upbuilding of sediment along the slope (Fig. 7 .l7b). The basin-floor fan
and slope fans are usually associated with canyon erosion into the slope and fluvial valley incision on the
shelf, particularly in its inner portion where subsidence
is slow. The base ofthe submarine fan is the lower sequence boundary (SB2). The slope fan is characterized
by turbidite and debris flow deposition. It may be coeval with the basin-floor fan or the early lowstand
wedge. In front of a river mouth, a lowstand delta may
be built out into the deeper sea. Towards the end of
LST, the sediments ofthe lowstand wedge onlap onto
the underlying sequence boundary, while the shoreline
still migrates seaward or becomes more or less stable.
N ow most of the incoming sediment is deposited on the
outer shelf (in shallow water). Simultaneously, incised
valleys on the shelf begin to be filled with fluvial,
estuarine, or shallow-marine sediments. The top ofthe
lowstand wedge is a marine flooding surface called the
"transgressive surface" (TS), which marks rapid landward retreat of the shoreline.
The subdivision ofLST into a basin floor fan, a slope fan, and
a lowstand wedge is a theoretical concept which is not realized everywhere. In Pleistocene sediments of a foreare basin
ofthe Boso Peninsula, Japan, for example, these subunits of
LST could not be identified (Ito 1998).
If lowstand deposits mainly lie on a Type 2 sequence
boundary (RSLF
relative sea -level fall occurs only along the coast and on
the inner shelf. Then the shoreline migrates rapidly
basinward and eroded sediment, including that of incised streams, is bypassed into deeper water. When the
sea level starts to rise slowly, incised valleys are filled
and a relatively thin lowstand wedge can be built out
seaward (cf. Fig. 7.l7b).
321
- The transgressive systems tract (TST) forms during
accelerated relative sea-Ievel rise when the incoming
sediment cannot further maintain the outbuilding of a
sediment wedge. The TST begins with the transgressive
surface (TS) and its strata onlap landward onto the
lower sequence boundary (Fig. 7.l7a, SBl); seaward
they downlap onto the transgressive surface. In Figure
7 .18a,b the TST is subdivided into parasequences characterized by transgressive lags at their base and
prograding wedges of coastal sands generating
shallowing-upward vertical sections. The top ofTST is
marked by the surface of maximum flooding (MFS)
where the stratal pattern changes from a landwardonlapping and upbuilding sedimentary unit to a
basinward-prograding wedge with downlap onto MFS
(downlap surface, DLS).
- At some distance basinward from the shoreline, the
time ofrnaximum flooding also signifies a time ofrnaximum sediment starvation. The resulting facies consist
of thin hemipelagic or pelagic beds deposited at very
low sedimentation rates. Such acondensed seetion (SC)
occurs largely within the transgressive and distal
highstand deposits (Loutit et al. 1988).
- The highstand systems tract (RS T) represents the late
part of a eustatic sea-level rise, its standstill, and the
early part of its fall. The RST sediments prograde
basinward in relation to the terrestrial sediment supply
(Fig. 7 .18c). In seismic sections, highstand deposits are
characterized by a lower aggradational part succeeded
bya seaward progradational unit with downlap onto the
top of transgressive deposits (MFS). The subsequent
rapid sea-level fall generates a Type 1 or Type 2 sequence boundary on top ofthe highstand deposits.
Two total sequences, the upper one overlain by a shelf
margin wedge replacing the lowstand systems tract, are
presented in Fig. 7.17 a as a Wheeler diagram. This
clearly shows how the shoreline migrates back and forth
during a sea-Ievel cycle. This phenomenon is expressed
in many publications by the so-called coastal onlap
curve. This diagram also demonstrates the long time
interval represented by the slowly deposited condensed
section. This is not, as sometimes erroneously interpreted from other Wheeler diagrams, a stratigraphic
gap.
As mentioned above, all systems tracts may be subdivided into parasequences and parasequence sets. Individual parasequences are typically bounded by marine
flooding surfaces and their "correlative surfaces" (Van
Wagoner et al. 1990). They are progradational and their
beds shoal upward (Fig. 7.l8). A parasequence may
consist of smallerunits representing time periods on the
order ofthe Milankovitch frequency band (Sect. 7.1 and
7.9.2).
The chronostratigraphic block diagram in Figure 7.17a
shows the development oftwo sequences. Similar to the
models in Figure 7.15, it is assumed that the subsidence
rate increases from a hinge line toward the basin center.
The influx of terrigenous material is moderate and the
sediments are predominantly siliciclastic. During the
transgressive and early highstand phase, most of the
incoming sediment is trapped in shallow water and on
the shelf, while little material is transported into deeper
water. During falling sea level and lowstand, the locus
of major sediment accumulation is the slope and continental rise, where relatively high sedimentation rates are
reached. The systems tracts of a third order sequence
can be characterized as follows:
- The lowstand systems tract (LS T) develops during the
phase of rapid eustatic sea-level fall up to the early part
ofthe rise (Fig. 7.l7a). Its specific characteristics on
passive margins are a basin floor fan and several slope
fans overlain by a lowstand wedge. The latter results
from continued prograding and some upbuilding of sediment along the slope (Fig. 7 .l7b). The basin-floor fan
and slope fans are usually associated with canyon erosion into the slope and fluvial valley incision on the
shelf, particularly in its inner portion where subsidence
is slow. The base ofthe submarine fan is the lower sequence boundary (SB2). The slope fan is characterized
by turbidite and debris flow deposition. It may be coeval with the basin-floor fan or the early lowstand
wedge. In front of a river mouth, a lowstand delta may
be built out into the deeper sea. Towards the end of
LST, the sediments ofthe lowstand wedge onlap onto
the underlying sequence boundary, while the shoreline
still migrates seaward or becomes more or less stable.
N ow most of the incoming sediment is deposited on the
outer shelf (in shallow water). Simultaneously, incised
valleys on the shelf begin to be filled with fluvial,
estuarine, or shallow-marine sediments. The top ofthe
lowstand wedge is a marine flooding surface called the
"transgressive surface" (TS), which marks rapid landward retreat of the shoreline.
The subdivision ofLST into a basin floor fan, a slope fan, and
a lowstand wedge is a theoretical concept which is not realized everywhere. In Pleistocene sediments of a foreare basin
ofthe Boso Peninsula, Japan, for example, these subunits of
LST could not be identified (Ito 1998).
If lowstand deposits mainly lie on a Type 2 sequence
boundary (RSLF
relative sea -level fall occurs only along the coast and on
the inner shelf. Then the shoreline migrates rapidly
basinward and eroded sediment, including that of incised streams, is bypassed into deeper water. When the
sea level starts to rise slowly, incised valleys are filled
and a relatively thin lowstand wedge can be built out
seaward (cf. Fig. 7.l7b).
321
- The transgressive systems tract (TST) forms during
accelerated relative sea-Ievel rise when the incoming
sediment cannot further maintain the outbuilding of a
sediment wedge. The TST begins with the transgressive
surface (TS) and its strata onlap landward onto the
lower sequence boundary (Fig. 7.l7a, SBl); seaward
they downlap onto the transgressive surface. In Figure
7 .18a,b the TST is subdivided into parasequences characterized by transgressive lags at their base and
prograding wedges of coastal sands generating
shallowing-upward vertical sections. The top ofTST is
marked by the surface of maximum flooding (MFS)
where the stratal pattern changes from a landwardonlapping and upbuilding sedimentary unit to a
basinward-prograding wedge with downlap onto MFS
(downlap surface, DLS).
- At some distance basinward from the shoreline, the
time ofrnaximum flooding also signifies a time ofrnaximum sediment starvation. The resulting facies consist
of thin hemipelagic or pelagic beds deposited at very
low sedimentation rates. Such acondensed seetion (SC)
occurs largely within the transgressive and distal
highstand deposits (Loutit et al. 1988).
- The highstand systems tract (RS T) represents the late
part of a eustatic sea-level rise, its standstill, and the
early part of its fall. The RST sediments prograde
basinward in relation to the terrestrial sediment supply
(Fig. 7 .18c). In seismic sections, highstand deposits are
characterized by a lower aggradational part succeeded
bya seaward progradational unit with downlap onto the
top of transgressive deposits (MFS). The subsequent
rapid sea-level fall generates a Type 1 or Type 2 sequence boundary on top ofthe highstand deposits.
Two total sequences, the upper one overlain by a shelf
margin wedge replacing the lowstand systems tract, are
presented in Fig. 7.17 a as a Wheeler diagram. This
clearly shows how the shoreline migrates back and forth
during a sea-Ievel cycle. This phenomenon is expressed
in many publications by the so-called coastal onlap
curve. This diagram also demonstrates the long time
interval represented by the slowly deposited condensed
section. This is not, as sometimes erroneously interpreted from other Wheeler diagrams, a stratigraphic
gap.
As mentioned above, all systems tracts may be subdivided into parasequences and parasequence sets. Individual parasequences are typically bounded by marine
flooding surfaces and their "correlative surfaces" (Van
Wagoner et al. 1990). They are progradational and their
beds shoal upward (Fig. 7.l8). A parasequence may
consist of smallerunits representing time periods on the
order ofthe Milankovitch frequency band (Sect. 7.1 and
7.9.2).