7.5 Marine Carbonate Systems
ups. The mass ofbenthic carbonate produced on a platform top during a sequence cyele of2 Ma, for example,
is on the order of 0.1 km 3 /km2, assuming a potential
vertical growth rate of 50 mrnJka = 50 m/Ma (cf. Figs.
3.29 and 10.4). Normally, this mass cannot be accommodated entirely by vertica1 aggradation of the platform, because subsidence and ACC are 1imited. As a
result, part of the excess carbonate mass is used for
prograding of the p1atform rim or exported into adj oining deeper areas.
The large-scale deve10pment of rimined p1atforms is
indicated in Figure 7.21b. For these models it is assumed thatthe rim is not very pronounced and the backreef zone more or less keeps up with sea level. During
TST and ear1y HST, upbuilding ofthe platform at increasing (TST) and decreasing rates (HST) predorninates. In particular TS Tappears to support reef growth,
because terrigenous input is subdued during this phase.
The variation in the vertical growth rates can be seen in
fie1d sections when the 3rd order cycles are superimposed by shorter cyeles (see below). During the late
HST the platform tends to prograde. If the sea level
drops below the platform rim the carbonate buildup is
exposed to subaerial weathering and karstification.
Rivers may cut valleys into the platform and transport
silicielastic material to the fore-reef slope. This in turn
hampers reef growth. However, carbonate production
can continue along the fore-reef slope. If this is steep,
only a narrow fringing reef can form. If the slope is flat
(muddy slopes), continued shallow-water carbonate
production may generate an autochthonous, widely
prograding lowstand wedge (e.g. Hunt and Tucker
1993; cf. Sect. 7.5.5). With sufficient subsidence atthe
platform edge, the lowstand systems tract is replaced by
a prograding shelf margin wedge.
A somewhat different and more detailed model of a
land-attached carbonate platform with some silicielastic
input is presented in Figure 7.22. When the inner platform is infilled and the prograding deposits of the late
HST or early LST have reached the former shelfbreak,
reef debris and reworked shelf carbonate are transported to the foot of the slope or basin floor by mass
flows and turbidity currents (calciturbidites or allodapic
limestones, cf. Sect. 5.4). Later, the activated terrigenous sediment source tends to generate mixed
carbonate-silicielastic or pure silicielastic slope and
basin sediments. In the absence ofterrigenous sediment
sources, relatively thick, possibly cyelic pelagic limestones and marls may accumulate in deep water, as the
calcite compensation depth (CCD; cf. Sect. 5.3.2) is
depressed during low sea levels. Moderate carbonate
aggradation, the absence ofterrigenous influx, and the
presence of siliceous sponges (as in the Jurassic and
Cretaceous ofEurope) often lead to the early diagenetic
formation of chert nodules within massive or bedded
limestones.
During subsequent transgressive and early highstand
phases, the input of terrigenous material into deeper
329
water is reduced. As a result, the condensed section in
deeper water is frequently represented by marls and
pelagic oozes (limestones ), which may contain high
proportions of organic carbon. At water depths below
the CCD, thin elay beds with some authigenic minerals
or black shales may develop.
Ifterrigenous sediment input mainly consists of mud,
then a basin of medium depth adjacent to a carbonate
shelf can display the following sediment succession (as,
for example, a 800 m thick Turonian limestone-marl
succession in central Tunisia; Robaszynski et al. 1990):
- HST, hernipelagic marl (dilution of pelagic carbonate
by terrigenous mud, low input of detrital carbonate
from shelt),
- TST, pelagic carbonate (low input of both terrigenous mud and detrital carbonate from shelt),
- LST, skeletal wackestones, packstones and grainstones (derived from shelt) with some hemipelagic
marI.
Steep slopes and escarpments of carbonate platforms
favor slope instability and platform collapse during
falling sea level and thus the formation of large debris
flows. If part of the carbonate was already indurated or
serni-lithified prior to failure, the mass flow deposits
have the appearance of sedimentary megabreccias containing gravel-size and even boulder-size elasts with
little matrix. Due to continued aggradation, carbonate
escarpment can also gain in height when prograding of
their rims is prevented by slopes too steep for deposition ofreeftalus or coarse skeletal elasts (Fig. 7.21c).
The rate of upbuilding increases during TST and decreases during HST. Correspondingly, the nature of
the foot-of-slope sediments of attached platforms often
changes from material dominated either by carbonate
or silicielastics.
7.5.3 Third-Order Sequences of
Carbonate Ramps
The response of carbonate ramps to sea-level changes
is similar to that of silicielastic systems. Solid structures such as reefs are mostly absent, sand barriers are
replaced by skeletal shoals, the backbarrier zone is
filled with tidal flat and lagoonal sediments mostly rich
in carbonate, and the ramp seaward of the marginal
shoals accumulates mainly skeletal debris and calcareous mud (cf. Sect. 3.4). Cool-water carbonates, in particular, frequently follow the silicielastic ramp model
because they lack distinct bioherms and are little affected by early lithification. Figure 7.23 provides an
overview of the 3rd order systems tracts of carbonate
ramps:
- TST, backstepping facies tracts consisting of marginal skeletal shoals which separate backbarrier
ups. The mass ofbenthic carbonate produced on a platform top during a sequence cyele of2 Ma, for example,
is on the order of 0.1 km 3 /km2, assuming a potential
vertical growth rate of 50 mrnJka = 50 m/Ma (cf. Figs.
3.29 and 10.4). Normally, this mass cannot be accommodated entirely by vertica1 aggradation of the platform, because subsidence and ACC are 1imited. As a
result, part of the excess carbonate mass is used for
prograding of the p1atform rim or exported into adj oining deeper areas.
The large-scale deve10pment of rimined p1atforms is
indicated in Figure 7.21b. For these models it is assumed thatthe rim is not very pronounced and the backreef zone more or less keeps up with sea level. During
TST and ear1y HST, upbuilding ofthe platform at increasing (TST) and decreasing rates (HST) predorninates. In particular TS Tappears to support reef growth,
because terrigenous input is subdued during this phase.
The variation in the vertical growth rates can be seen in
fie1d sections when the 3rd order cycles are superimposed by shorter cyeles (see below). During the late
HST the platform tends to prograde. If the sea level
drops below the platform rim the carbonate buildup is
exposed to subaerial weathering and karstification.
Rivers may cut valleys into the platform and transport
silicielastic material to the fore-reef slope. This in turn
hampers reef growth. However, carbonate production
can continue along the fore-reef slope. If this is steep,
only a narrow fringing reef can form. If the slope is flat
(muddy slopes), continued shallow-water carbonate
production may generate an autochthonous, widely
prograding lowstand wedge (e.g. Hunt and Tucker
1993; cf. Sect. 7.5.5). With sufficient subsidence atthe
platform edge, the lowstand systems tract is replaced by
a prograding shelf margin wedge.
A somewhat different and more detailed model of a
land-attached carbonate platform with some silicielastic
input is presented in Figure 7.22. When the inner platform is infilled and the prograding deposits of the late
HST or early LST have reached the former shelfbreak,
reef debris and reworked shelf carbonate are transported to the foot of the slope or basin floor by mass
flows and turbidity currents (calciturbidites or allodapic
limestones, cf. Sect. 5.4). Later, the activated terrigenous sediment source tends to generate mixed
carbonate-silicielastic or pure silicielastic slope and
basin sediments. In the absence ofterrigenous sediment
sources, relatively thick, possibly cyelic pelagic limestones and marls may accumulate in deep water, as the
calcite compensation depth (CCD; cf. Sect. 5.3.2) is
depressed during low sea levels. Moderate carbonate
aggradation, the absence ofterrigenous influx, and the
presence of siliceous sponges (as in the Jurassic and
Cretaceous ofEurope) often lead to the early diagenetic
formation of chert nodules within massive or bedded
limestones.
During subsequent transgressive and early highstand
phases, the input of terrigenous material into deeper
329
water is reduced. As a result, the condensed section in
deeper water is frequently represented by marls and
pelagic oozes (limestones ), which may contain high
proportions of organic carbon. At water depths below
the CCD, thin elay beds with some authigenic minerals
or black shales may develop.
Ifterrigenous sediment input mainly consists of mud,
then a basin of medium depth adjacent to a carbonate
shelf can display the following sediment succession (as,
for example, a 800 m thick Turonian limestone-marl
succession in central Tunisia; Robaszynski et al. 1990):
- HST, hernipelagic marl (dilution of pelagic carbonate
by terrigenous mud, low input of detrital carbonate
from shelt),
- TST, pelagic carbonate (low input of both terrigenous mud and detrital carbonate from shelt),
- LST, skeletal wackestones, packstones and grainstones (derived from shelt) with some hemipelagic
marI.
Steep slopes and escarpments of carbonate platforms
favor slope instability and platform collapse during
falling sea level and thus the formation of large debris
flows. If part of the carbonate was already indurated or
serni-lithified prior to failure, the mass flow deposits
have the appearance of sedimentary megabreccias containing gravel-size and even boulder-size elasts with
little matrix. Due to continued aggradation, carbonate
escarpment can also gain in height when prograding of
their rims is prevented by slopes too steep for deposition ofreeftalus or coarse skeletal elasts (Fig. 7.21c).
The rate of upbuilding increases during TST and decreases during HST. Correspondingly, the nature of
the foot-of-slope sediments of attached platforms often
changes from material dominated either by carbonate
or silicielastics.
7.5.3 Third-Order Sequences of
Carbonate Ramps
The response of carbonate ramps to sea-level changes
is similar to that of silicielastic systems. Solid structures such as reefs are mostly absent, sand barriers are
replaced by skeletal shoals, the backbarrier zone is
filled with tidal flat and lagoonal sediments mostly rich
in carbonate, and the ramp seaward of the marginal
shoals accumulates mainly skeletal debris and calcareous mud (cf. Sect. 3.4). Cool-water carbonates, in particular, frequently follow the silicielastic ramp model
because they lack distinct bioherms and are little affected by early lithification. Figure 7.23 provides an
overview of the 3rd order systems tracts of carbonate
ramps:
- TST, backstepping facies tracts consisting of marginal skeletal shoals which separate backbarrier