7.5 Marine Carbonate Systems
The western flank of the Great Bahama Bank including the
foot of the slope was recently investigated by a number of
drillholes (ODP Leg 166; Eberli et al. 1997; Betzler et al.
1999). The Neogene sediments show a light-grey/dark-grey
wackestone/packstone cyclicity controlled by hlgh-frequency
sea-Ievel changes. The dark-grey layers ar.e dominate~ by
pelagic material. The interbedded calcIturbldltes and mmor
slumps fonn mounded lobes and are associated ,,:ith fee.der
channels cut into the slope. It appears that the calclturbldItes
were shed during both third-order highstands and 10'Ystand~.
In the latter case the turbidites consist of abraded blOc1astIc
detritus. At the' foot of the slope this facies association
interfingers with drift sediments ofthe Straits ofFlorida.
The slopes of land-attached structures commonly exhibit both platform-derived skeletal detritus and landderived siliciclastic material. Figure 7.24b provides a
simplified facies model of this situation. Ac.tually, the
slope sediments of attached carbonate bUlldups are
often more complex.
Carbonates generally originate from both lateral influx from the platform top and some benthic carbonate
produced on the slope, as well as. planktic carbon~te
settling from suspension (pelaglc carbonate; Flg.
7.24c). The platform-derived sed~ment component n~rmally dominates the upper and mlddle slope and exhIbits decreasing grain sizes downslope (e.g. from gra~elsize material to fine-grained skeletal calcaremte).
Basinward, hemipelagic and pelagic marls and ooz~s,
including some fine-grained siliciclastics, bec0lll:e mcreasingly important. In addition, storms can stlr up
platform mud and generate suspensions of s~mewhat
increased density (hyperpycnal waters) WhlCh flow
into adjacent deeper basins ("densitycasc.ading" after
Wilson and Roberts 1995). The denslty currents
hereby pass the upper slope and cause a kin? of "pelagic draping" in deeper water. Larger gravlty flo:"s
and turbidity currents episodically transport matenal
from the platform edge and upper slope into deeper
water. Both debris flow deposits and turbidites can
consist of mixed carbonate-siliciclastic components
(sandy turbidites and calciturbidites). Rock falls and
platform margin collapse generate debris flows ~esulting in carbonate breccias and conglomerates WhlCh are
either clast- or matrix-supported.
This generalized scenario is modified by relative
sea-Ievel changes, as shown in Figure 7.24d for a mor.e
or less vertically aggrading attached platform expenencing one cycle of sea-Ievel fall and rise. The different stages of evolution are briefly described as follows:
(1) and (5) Highstand (too high for photozoan associations, cf. Sect. 3.4), limited production and export of
platform carbonate to slope; therefore accumulation of
hemipelagic marl and pelagic lime mud on the deep
slope.
(2) Lowering sea level, increasing production .and export ofplatform carbonate to slope; some debns flows
and calciturbidites.
333
(3) Sea level has dropped below p.latfo~ e.dge: ~alley
incision and bypassing ofland-denved stllclclastlc material. Rock falls and slope failures at platform edge
and uppermost slope generate calcareous megabreccias
(olistostromes, i.e., a kind of basin-floor fan) at the
lower slope. Simultaneously or shortly later,. ?y'pass~d
siliciclastics may form silty clays and slhclclastlc
turbidites. The potential growth of a narrow lowstand
fringing reef is omitted in this model. .
(4) Rising sea level, renewed productlon of platform
carbonate and its export to the slope.
In all stages of this development the slope facies
changes laterally as indicated above. P.latform c?llapse
seerns to be not always associated wlth lowenng sea
level (Bosellini et al. 1993) and oversteep~ned sl~pes
are not generally necessary for the formatiOn of hmestone megabreccias (Spence and Tucker 1997).
Many carbonate platforms pro grade basinward, because their carbonate production, GR, is higher than
the sediment accommodation space, ACC, provided by
subsidence (e.g. that of rift and continental margin basins during their intermediate and lat~ p~ases o.f evo.lution; cf. Chap. 8). Platform progradmg m conJunctlon
with early cementation tends to produce steep forereef
slopes and thus coarse reef talus and larg~ se~iment
gravity flows. This state is more or less mamtamed .as
long as the carbonate buildup can keep up well wl!h
relative SL rise. This also applies to the shelf margm
systems tract, SMW, as long as rel~tiv~ SL ~s st.ill
slowly rising on the outer platform as mdlcated m Flg.
7.24d. Then specific LS-sediments cannot form on the
slope. By contrast, relatively fast SL-rise, for example
during the late TST, will cause backstepping of the
platform edge and thus significantl~ reduce talus production and the occurrence of gravlty flows.
When RSL falls deeper than the platform top and
carbonate production can be maintained on ~ gentle
slope, the SL change is testified by downstep~mg narrow terraces (forced regression). Mass wastmg may
produce a lowstand (LS) basin-floor fan compo~ed of
allochthonous material, as described above. Contmued
reworking of platform carbonate and/or influx oflan.dderived siliciclastics can generate aLS slope fan. If ItS
slope angle is low (mud-rich mater~al) an~ the late
lowstand lasts a sufficiently long penod of tlme, benthic carbonate production on the slope can be substantial and create an autochthonous, prograding wide LS
wedge (Fig. 7.24d). An alternative to this model is the
prograding of a siliciclastic or mixed LS wedge.
Lowstand prograding mayaiso occur in distinct steps,
when a long-term trend of SL fall is superimposed by
higher frequency SL fluctuations (Fig. 7.24e).
These few scenarios cannot describe all types of slope facies
occurring in nature with or without the influence of sea-Ievel
changes (see, e.g., Hunt and Tucker 1993; Southgate et al.
1993; Mazullo 1995; Mutti et al. 1996; James et al. 1997;
Osleger 1998), but they may be useful for a better under-
The western flank of the Great Bahama Bank including the
foot of the slope was recently investigated by a number of
drillholes (ODP Leg 166; Eberli et al. 1997; Betzler et al.
1999). The Neogene sediments show a light-grey/dark-grey
wackestone/packstone cyclicity controlled by hlgh-frequency
sea-Ievel changes. The dark-grey layers ar.e dominate~ by
pelagic material. The interbedded calcIturbldltes and mmor
slumps fonn mounded lobes and are associated ,,:ith fee.der
channels cut into the slope. It appears that the calclturbldItes
were shed during both third-order highstands and 10'Ystand~.
In the latter case the turbidites consist of abraded blOc1astIc
detritus. At the' foot of the slope this facies association
interfingers with drift sediments ofthe Straits ofFlorida.
The slopes of land-attached structures commonly exhibit both platform-derived skeletal detritus and landderived siliciclastic material. Figure 7.24b provides a
simplified facies model of this situation. Ac.tually, the
slope sediments of attached carbonate bUlldups are
often more complex.
Carbonates generally originate from both lateral influx from the platform top and some benthic carbonate
produced on the slope, as well as. planktic carbon~te
settling from suspension (pelaglc carbonate; Flg.
7.24c). The platform-derived sed~ment component n~rmally dominates the upper and mlddle slope and exhIbits decreasing grain sizes downslope (e.g. from gra~elsize material to fine-grained skeletal calcaremte).
Basinward, hemipelagic and pelagic marls and ooz~s,
including some fine-grained siliciclastics, bec0lll:e mcreasingly important. In addition, storms can stlr up
platform mud and generate suspensions of s~mewhat
increased density (hyperpycnal waters) WhlCh flow
into adjacent deeper basins ("densitycasc.ading" after
Wilson and Roberts 1995). The denslty currents
hereby pass the upper slope and cause a kin? of "pelagic draping" in deeper water. Larger gravlty flo:"s
and turbidity currents episodically transport matenal
from the platform edge and upper slope into deeper
water. Both debris flow deposits and turbidites can
consist of mixed carbonate-siliciclastic components
(sandy turbidites and calciturbidites). Rock falls and
platform margin collapse generate debris flows ~esulting in carbonate breccias and conglomerates WhlCh are
either clast- or matrix-supported.
This generalized scenario is modified by relative
sea-Ievel changes, as shown in Figure 7.24d for a mor.e
or less vertically aggrading attached platform expenencing one cycle of sea-Ievel fall and rise. The different stages of evolution are briefly described as follows:
(1) and (5) Highstand (too high for photozoan associations, cf. Sect. 3.4), limited production and export of
platform carbonate to slope; therefore accumulation of
hemipelagic marl and pelagic lime mud on the deep
slope.
(2) Lowering sea level, increasing production .and export ofplatform carbonate to slope; some debns flows
and calciturbidites.
333
(3) Sea level has dropped below p.latfo~ e.dge: ~alley
incision and bypassing ofland-denved stllclclastlc material. Rock falls and slope failures at platform edge
and uppermost slope generate calcareous megabreccias
(olistostromes, i.e., a kind of basin-floor fan) at the
lower slope. Simultaneously or shortly later,. ?y'pass~d
siliciclastics may form silty clays and slhclclastlc
turbidites. The potential growth of a narrow lowstand
fringing reef is omitted in this model. .
(4) Rising sea level, renewed productlon of platform
carbonate and its export to the slope.
In all stages of this development the slope facies
changes laterally as indicated above. P.latform c?llapse
seerns to be not always associated wlth lowenng sea
level (Bosellini et al. 1993) and oversteep~ned sl~pes
are not generally necessary for the formatiOn of hmestone megabreccias (Spence and Tucker 1997).
Many carbonate platforms pro grade basinward, because their carbonate production, GR, is higher than
the sediment accommodation space, ACC, provided by
subsidence (e.g. that of rift and continental margin basins during their intermediate and lat~ p~ases o.f evo.lution; cf. Chap. 8). Platform progradmg m conJunctlon
with early cementation tends to produce steep forereef
slopes and thus coarse reef talus and larg~ se~iment
gravity flows. This state is more or less mamtamed .as
long as the carbonate buildup can keep up well wl!h
relative SL rise. This also applies to the shelf margm
systems tract, SMW, as long as rel~tiv~ SL ~s st.ill
slowly rising on the outer platform as mdlcated m Flg.
7.24d. Then specific LS-sediments cannot form on the
slope. By contrast, relatively fast SL-rise, for example
during the late TST, will cause backstepping of the
platform edge and thus significantl~ reduce talus production and the occurrence of gravlty flows.
When RSL falls deeper than the platform top and
carbonate production can be maintained on ~ gentle
slope, the SL change is testified by downstep~mg narrow terraces (forced regression). Mass wastmg may
produce a lowstand (LS) basin-floor fan compo~ed of
allochthonous material, as described above. Contmued
reworking of platform carbonate and/or influx oflan.dderived siliciclastics can generate aLS slope fan. If ItS
slope angle is low (mud-rich mater~al) an~ the late
lowstand lasts a sufficiently long penod of tlme, benthic carbonate production on the slope can be substantial and create an autochthonous, prograding wide LS
wedge (Fig. 7.24d). An alternative to this model is the
prograding of a siliciclastic or mixed LS wedge.
Lowstand prograding mayaiso occur in distinct steps,
when a long-term trend of SL fall is superimposed by
higher frequency SL fluctuations (Fig. 7.24e).
These few scenarios cannot describe all types of slope facies
occurring in nature with or without the influence of sea-Ievel
changes (see, e.g., Hunt and Tucker 1993; Southgate et al.
1993; Mazullo 1995; Mutti et al. 1996; James et al. 1997;
Osleger 1998), but they may be useful for a better under-