7.5 Marine Carbonate Systems
standing of the general processes controlling this complex
depositional environment.
Slope sediments can also record high-frequency sea-level
changes if their HS and LS sediments distantly differ in primary composition (calcitic versus aragonitic skeletal material) and diagenesis, as, e.g., in the Pliocene of the western
slope ofthe Bahama Platform (Westphal 1998) and in Aptian
carbonates of a ramp margin in NE Spain (Bachmann and
Willems 1996).
7.5.6 Carbonate-Evaporite Systems
In classical models of carbonate-evaporite sequences
of large intracratonic basins, evaporite successions
were mainly studied and interpreted in the basin centers. The water level in the basin could either remain ±
constant, or it was lowered by evaporative drawdown
(cf. Sects. 2.5 and 11.4). Correlation of marginal with
basinal sediments mostly remained unclear. The traditional view did not sufficiently take into account that
along basin margins sediment successions may form
which are different from coevel sediments in the basin
center. This problem can be overcome by a sequence
stratigraphic approach to carbonate-evaporite basins.
The modified model (Tucker 1991) is particularly useful for
large and rapidly subsiding basins (giant salt deposits) such
as the Upper Permian Zechstein basin of northwestem Europe and the North Sea (cf. Sect. 6.4.2 and Figs. 6.9 and
6.10), several salt-bearing intracratonic basins in North
America, and the basins of Moscow and Siberia. A more
detailed and somewhat deviating view for the Gerrnan part of
the Zechstein Basin was proposed by Strohmenger et al.
(1996a).
The first stage of the improved facies model depicts a
semi-closed basin during sea-Ievel highstand with little
siliciclastic input (Fig. 7.25a). Due to water exchange
with the open sea the salinity of the basin is normal or
slightly hypersaline, still allowing the growth of reefs
and mud mounds and relatively rapid biogenic carbonate deposition along the basin margin. In the basin center, pelagic to hemipelagic lime mud (± bituminous and
laminated) accumulates. The following basin closure
and evaporative drawdown of the water level causes
Fig. 7.24. Reef foreslope deposits in carbonate and
mixed carbonate-siliciclastic systems. a Slope deposits of late Pleistocene SL-Iowstand and Holocene SLhighstand at isolated platform (Bahamas/Tongue of
the Ocean; after Grammer and Ginsburg 1992).
b Third order sequences of prograding carbonate
shelf with siliciclastic lowstand wedges (after Sarg
1988). c General processes of slope sedimentation
along land-attached carbonate platform with highangle slope and subdued LS-deposits. Hypothetical
facies change during one SL-cycle from highstand to
highstand. d Slope sediments of sea-Ievel controlled
prograding and backstepping rimmed carbonate shelf
335
increased salinity and deposition of inorganic carbonate (later transformed to dolomite) and sabkha gypsum
on the emerging shelf. As the water level continues to
fall, gypsum accumulates on the slope (partially as
selenitic gypsum) where it can form a thick
downstepping wedge (Fig. 7.25b). Some of the gypsum is redeposited by debris flows and turbidity currents to form breccias and graded beds in the deeper
basin. Simultaneously, laminated gypsum and carbonate muds, often rich in organic matter, accumulate in
the basin center.
The further development is mainly controlled by the
degree of drawdown, i.e. by the amount of renewed
influx of sea water over the sill, seepage of sea water
through permeable rocks, or change to a more humid
or arid climate. When drawdown is incomplete, highly
soluble salts such as halite cannot be precipitated, but
gypsum continues to form thick and extensive deposits
at the basin margin. A subsequently rising water level
(TS T) allows the precipitation of sabkha evaporites on
the platform before carbonate deposition due to lowered salinity takes over in the entire basin (HST, Fig.
7.25c and d). Under these conditions, the resulting idealized sequence on the shelf begins on top of a
subaerial hiatus (representing LST) and consists of
sabkha evaporites grading upward into shallow-water
carbonates which are overlain again by sabkha
evaporites (TST and HST).
Complete drawdown ofthe water level (LST) brings
about supersaturation with respect to halite and thus
halite precipitation from ponds and shallow salt lakes
(Fig. 7.25e). Halite can form in large quantities and
locally also some potash salts when the salt concentration in the water body was sufficiently high prior to
drawdown (cf. Sect. 6.4) and/or salt influx is maintained by seepage of sea water. Then the basin can be
largely and rapidly filled up with rock salto
Renewed opening of the basin to the world ocean in
conjunction with a general sea-Ievelrise or local tectonics, affecting the sill area, again leads to a waterlevel rise in the basin. During early TST, carbonate
deposition may begin in the basin center while
evaporites are still being precipitated along the shallower basin margin. A further decrease in salinity durwith low-angle slope. During the formation of a shelf
margin wedge, SMW, prograding continues more or
less as during HST. When SL falls below platform
rim, an autochthonous LS-wedge can prograde on top
of a basin-floor fan and slope fan. Depending on the
sediment source and climate, the LS-deposits can be
either siliciclastic, mixed carbonate/siliciclastics, or
predominantly redeposited and autochthonous carbonates. (Based on Hunt and Tucker 1993; Southgate
et al. 1993). e Downstepping lowstand wedges reflecting high-frequency SL-changes during long-term
trend of falling relative SL. (After Mutti et al. 1996,
SL-curve supplemented)
standing of the general processes controlling this complex
depositional environment.
Slope sediments can also record high-frequency sea-level
changes if their HS and LS sediments distantly differ in primary composition (calcitic versus aragonitic skeletal material) and diagenesis, as, e.g., in the Pliocene of the western
slope ofthe Bahama Platform (Westphal 1998) and in Aptian
carbonates of a ramp margin in NE Spain (Bachmann and
Willems 1996).
7.5.6 Carbonate-Evaporite Systems
In classical models of carbonate-evaporite sequences
of large intracratonic basins, evaporite successions
were mainly studied and interpreted in the basin centers. The water level in the basin could either remain ±
constant, or it was lowered by evaporative drawdown
(cf. Sects. 2.5 and 11.4). Correlation of marginal with
basinal sediments mostly remained unclear. The traditional view did not sufficiently take into account that
along basin margins sediment successions may form
which are different from coevel sediments in the basin
center. This problem can be overcome by a sequence
stratigraphic approach to carbonate-evaporite basins.
The modified model (Tucker 1991) is particularly useful for
large and rapidly subsiding basins (giant salt deposits) such
as the Upper Permian Zechstein basin of northwestem Europe and the North Sea (cf. Sect. 6.4.2 and Figs. 6.9 and
6.10), several salt-bearing intracratonic basins in North
America, and the basins of Moscow and Siberia. A more
detailed and somewhat deviating view for the Gerrnan part of
the Zechstein Basin was proposed by Strohmenger et al.
(1996a).
The first stage of the improved facies model depicts a
semi-closed basin during sea-Ievel highstand with little
siliciclastic input (Fig. 7.25a). Due to water exchange
with the open sea the salinity of the basin is normal or
slightly hypersaline, still allowing the growth of reefs
and mud mounds and relatively rapid biogenic carbonate deposition along the basin margin. In the basin center, pelagic to hemipelagic lime mud (± bituminous and
laminated) accumulates. The following basin closure
and evaporative drawdown of the water level causes
Fig. 7.24. Reef foreslope deposits in carbonate and
mixed carbonate-siliciclastic systems. a Slope deposits of late Pleistocene SL-Iowstand and Holocene SLhighstand at isolated platform (Bahamas/Tongue of
the Ocean; after Grammer and Ginsburg 1992).
b Third order sequences of prograding carbonate
shelf with siliciclastic lowstand wedges (after Sarg
1988). c General processes of slope sedimentation
along land-attached carbonate platform with highangle slope and subdued LS-deposits. Hypothetical
facies change during one SL-cycle from highstand to
highstand. d Slope sediments of sea-Ievel controlled
prograding and backstepping rimmed carbonate shelf
335
increased salinity and deposition of inorganic carbonate (later transformed to dolomite) and sabkha gypsum
on the emerging shelf. As the water level continues to
fall, gypsum accumulates on the slope (partially as
selenitic gypsum) where it can form a thick
downstepping wedge (Fig. 7.25b). Some of the gypsum is redeposited by debris flows and turbidity currents to form breccias and graded beds in the deeper
basin. Simultaneously, laminated gypsum and carbonate muds, often rich in organic matter, accumulate in
the basin center.
The further development is mainly controlled by the
degree of drawdown, i.e. by the amount of renewed
influx of sea water over the sill, seepage of sea water
through permeable rocks, or change to a more humid
or arid climate. When drawdown is incomplete, highly
soluble salts such as halite cannot be precipitated, but
gypsum continues to form thick and extensive deposits
at the basin margin. A subsequently rising water level
(TS T) allows the precipitation of sabkha evaporites on
the platform before carbonate deposition due to lowered salinity takes over in the entire basin (HST, Fig.
7.25c and d). Under these conditions, the resulting idealized sequence on the shelf begins on top of a
subaerial hiatus (representing LST) and consists of
sabkha evaporites grading upward into shallow-water
carbonates which are overlain again by sabkha
evaporites (TST and HST).
Complete drawdown ofthe water level (LST) brings
about supersaturation with respect to halite and thus
halite precipitation from ponds and shallow salt lakes
(Fig. 7.25e). Halite can form in large quantities and
locally also some potash salts when the salt concentration in the water body was sufficiently high prior to
drawdown (cf. Sect. 6.4) and/or salt influx is maintained by seepage of sea water. Then the basin can be
largely and rapidly filled up with rock salto
Renewed opening of the basin to the world ocean in
conjunction with a general sea-Ievelrise or local tectonics, affecting the sill area, again leads to a waterlevel rise in the basin. During early TST, carbonate
deposition may begin in the basin center while
evaporites are still being precipitated along the shallower basin margin. A further decrease in salinity durwith low-angle slope. During the formation of a shelf
margin wedge, SMW, prograding continues more or
less as during HST. When SL falls below platform
rim, an autochthonous LS-wedge can prograde on top
of a basin-floor fan and slope fan. Depending on the
sediment source and climate, the LS-deposits can be
either siliciclastic, mixed carbonate/siliciclastics, or
predominantly redeposited and autochthonous carbonates. (Based on Hunt and Tucker 1993; Southgate
et al. 1993). e Downstepping lowstand wedges reflecting high-frequency SL-changes during long-term
trend of falling relative SL. (After Mutti et al. 1996,
SL-curve supplemented)