328
Chapter 7 Sequences, Minor Cycles, and Event Stratigraphy
7.4.3 Other Deep-Sea Sediments
Deep-sea sediments not affected bythe processes forming deep-sea fans or slope fans "feei" relative sea-Ievel
changes only indirectly. Their generally low sedimentation rate may increase during lowering and decrease
during rising sea levels. Rising sea levels can elevate
the calcite compensation depth and thus promote carbonate dissolution (e.g. Haq 1993). In addition, oxygen
supply to the deep sea may deteriorate during highstand.
On the other hand, lowstand can reduce water exchange
7.4.4 Summary (Deltas, Deep-Sea Fans, ete.)
- Cyclic phenomena in marine deltas originate
from both switching of delta lobes and sea-Ievel
changes.
- Delta prograding during sea-Ievel fall slows with
increasing water depth and decreasing sediment
supply. Prograding can also occur during slow
sea-Ievel rise if river-bome sediment influx is
high.
- Lowstand deltas on shelves have a good chance
of being preserved. If they have prograded to the
shelf edge, they generate submarine canyons and
feed deep-sea fans.
7.5 Sequence Stratigraphy of Marine
Carbonate Systems
7.5.1 General Aspeets
The various aspects of shallow-marine carbonate production and sediment buildup in warm and cool marine
environments, including their general behavior to relative rise or fall of sea level, are dealt with in Sect. 3.4.
Linear sedimentation rates of carbonates are summarized in Figure 3.29 and listed in Section 10.2 (cf. Figs.
10.3 and 10.4).
The fundamental difference between allochthonous
siliciclastic sediments and autochthonous carbonates,
produced in the depositional environment itself, also
controls to a large extent the response of marine carbonates and carbonate buildups to sea-Ievel changes. In
contrast to the highly variable, irregular siliciclastic
sediment influx into a basin, in-situ carbonate production on a platform is more predictable. When platforrns
are flooded (shallow water), benthic carbonate production, especially in the photic zone, is usually high (cf.
Figs.lO.3 and 10.4), butwhenpartof, ortheentireplatform emerges, the "carbonate factory" is more or less
tumed off. Relative high stands of sea-Ievel therefore
often lead to "highstand shedding" of platform carbonates into deeper water, but significant carbonate producthrough oceanic gateways and thus bring about significant changes in thermohaline circulation and nutrient
supply. It seems that deep bottom currents are often
more effective in terms of erosion and redistribution of
sediments during lowstands than during highstands. The
release of gas hydrates (methane) from continental
slope sediments as a result of reduced pressure has been
quoted by several authors to explain the causallink between submarine mass flows and lowered sea level (cf.
Sect. 14.3.6).
- High-frequency, high-amplitude sea-Ievel
changes tend to cause deep-sea fans to aggrade
and prograde during lowstands (channel-Ievee
complexes). However, not all fans become inactive and are covered by a hemipelagic drape
during highstands.
- Other deep-sea sediments may respond indirectly to sea-Ievel changes, e.g. by rise of the
CCD and accelerated bottom currents during
sea-Ievel fall, or by decreasing oxygenation of
bottom water during sea-Ievel rise.
tion can continue on gentle fore-reef slopes also during
sea-Ievellowstand.
With the aid of these general mIes it is possible to
simulate to some extent the architecture of calcareous
sedimentary bodies resulting from the interplay of sediment production, subsidence and sea-Ievel change. The
evident relationship between the change in sediment
accommodation space, ACC, controlled by subsidence
and sea level, and carbonate production, GR, on platform tops is shown in Figure 7.21a. With GR>ACC,
the platform rimprogrades basinward; with GR
level. When carbonate production at the platform rim
is only sufficient to follow relative sea-Ievel rise, the
central part of the platform top is transformed into a
deepening lagoon, etc.
These aspects are discussed in more detail, e.g., by Schlager
(1993, 1994), Hunt and Tucker (1993) and many others (see
below). Carbonate ramp depositional systems are described
byTucker and Wright (1990), Burchette and Wright (1992).
7.5.2 Third-Order Sequenees of Rimmed
Carbonate Platforms
Low-frequency sea-Ievel changes (2nd and 3rd order)
create large-scale architectural styles of carbonate build-
Chapter 7 Sequences, Minor Cycles, and Event Stratigraphy
7.4.3 Other Deep-Sea Sediments
Deep-sea sediments not affected bythe processes forming deep-sea fans or slope fans "feei" relative sea-Ievel
changes only indirectly. Their generally low sedimentation rate may increase during lowering and decrease
during rising sea levels. Rising sea levels can elevate
the calcite compensation depth and thus promote carbonate dissolution (e.g. Haq 1993). In addition, oxygen
supply to the deep sea may deteriorate during highstand.
On the other hand, lowstand can reduce water exchange
7.4.4 Summary (Deltas, Deep-Sea Fans, ete.)
- Cyclic phenomena in marine deltas originate
from both switching of delta lobes and sea-Ievel
changes.
- Delta prograding during sea-Ievel fall slows with
increasing water depth and decreasing sediment
supply. Prograding can also occur during slow
sea-Ievel rise if river-bome sediment influx is
high.
- Lowstand deltas on shelves have a good chance
of being preserved. If they have prograded to the
shelf edge, they generate submarine canyons and
feed deep-sea fans.
7.5 Sequence Stratigraphy of Marine
Carbonate Systems
7.5.1 General Aspeets
The various aspects of shallow-marine carbonate production and sediment buildup in warm and cool marine
environments, including their general behavior to relative rise or fall of sea level, are dealt with in Sect. 3.4.
Linear sedimentation rates of carbonates are summarized in Figure 3.29 and listed in Section 10.2 (cf. Figs.
10.3 and 10.4).
The fundamental difference between allochthonous
siliciclastic sediments and autochthonous carbonates,
produced in the depositional environment itself, also
controls to a large extent the response of marine carbonates and carbonate buildups to sea-Ievel changes. In
contrast to the highly variable, irregular siliciclastic
sediment influx into a basin, in-situ carbonate production on a platform is more predictable. When platforrns
are flooded (shallow water), benthic carbonate production, especially in the photic zone, is usually high (cf.
Figs.lO.3 and 10.4), butwhenpartof, ortheentireplatform emerges, the "carbonate factory" is more or less
tumed off. Relative high stands of sea-Ievel therefore
often lead to "highstand shedding" of platform carbonates into deeper water, but significant carbonate producthrough oceanic gateways and thus bring about significant changes in thermohaline circulation and nutrient
supply. It seems that deep bottom currents are often
more effective in terms of erosion and redistribution of
sediments during lowstands than during highstands. The
release of gas hydrates (methane) from continental
slope sediments as a result of reduced pressure has been
quoted by several authors to explain the causallink between submarine mass flows and lowered sea level (cf.
Sect. 14.3.6).
- High-frequency, high-amplitude sea-Ievel
changes tend to cause deep-sea fans to aggrade
and prograde during lowstands (channel-Ievee
complexes). However, not all fans become inactive and are covered by a hemipelagic drape
during highstands.
- Other deep-sea sediments may respond indirectly to sea-Ievel changes, e.g. by rise of the
CCD and accelerated bottom currents during
sea-Ievel fall, or by decreasing oxygenation of
bottom water during sea-Ievel rise.
tion can continue on gentle fore-reef slopes also during
sea-Ievellowstand.
With the aid of these general mIes it is possible to
simulate to some extent the architecture of calcareous
sedimentary bodies resulting from the interplay of sediment production, subsidence and sea-Ievel change. The
evident relationship between the change in sediment
accommodation space, ACC, controlled by subsidence
and sea level, and carbonate production, GR, on platform tops is shown in Figure 7.21a. With GR>ACC,
the platform rimprogrades basinward; with GR
level. When carbonate production at the platform rim
is only sufficient to follow relative sea-Ievel rise, the
central part of the platform top is transformed into a
deepening lagoon, etc.
These aspects are discussed in more detail, e.g., by Schlager
(1993, 1994), Hunt and Tucker (1993) and many others (see
below). Carbonate ramp depositional systems are described
byTucker and Wright (1990), Burchette and Wright (1992).
7.5.2 Third-Order Sequenees of Rimmed
Carbonate Platforms
Low-frequency sea-Ievel changes (2nd and 3rd order)
create large-scale architectural styles of carbonate build-