7.6 Transitional Systems
ing late TST and HST causes carbonate accumulation
on both the shelf and basin center (Fig. 7.25f). This
could be more or less the end of the basin filling leaving behind one evaporite-carbonate sequence, but ongoing differential subsidence can more or less restore
the initial basin morphology and thus produce subsequent sequences similar to the first one (Fig. 7.25g).
In this concept, the sequences begin on the shelf
with an unconformity (SB I) on top of carbonates (or
sabkha evaporites ) and in the basin center with a correlative conformity (SB2) on top of hemipelagic thin
marls (± biturninous and larninated). The basinal
evaporites are precipitated in the LST and early TST.
They form thick basinal salt deposits (rnainly halite
and gypsum), a gypsum wedge along the shelf
foreslope, and some sabkha evaporites on the shelf.
7.5.7 Summary (Carbonate Systems, Evaporites)
- Shallow-marine carbonate systems fundamentally deviate from siliciclastic systems due to
their autochthonous, rnainly benthic carbonate
production.
- Vertical growth of carbonate buildups can normally keep up with sea-Ievel rise. Drowning of
carbonate platforrns is mostly associated with
deteriorated environmental conditions for carbonate production or extremely rapid sea-Ievel
nse.
- A surplus in carbonate production, in relation to
sediment accommodation space, leads to
"highstand shedding", the prograding of rimmed
carbonate platforms, and to extensive carbonate
slopes and carbonate "breccias".
- Carbonate ramps respond to sea-Ievel changes in
a similar way as siliciclastic ramp settings, but
7.6 Sequence Stratigraphy of
Transitional Systems
The transitional zone between shallow seas and coastal
lowlands plays a major role in sequence stratigraphy.
It is this zone where relative sea level changes, both
rise and fall, exert the most pronounced influence on
the facies and architecture of sediments. Many of these
transitional environments have already been mentioned
in the facies models of Sects. 7.3 to 7.5 and in Sects.
3.1,3.2 and 3.5. The following seetions focus on (1)
valleys incised during sea-Ievel fall and subsequently
filled with sediment as well as (2) on the repeated
flooding or emergence of coastal alluvial plains, lagoons and lakes.
337
Substantial amounts of shelf carbonates accumulate
during the late TST and early HST when the basin is
flooded. When the water level starts to fall again (late
HST), shallow lagoons and sabkhas can be re-established in the landward portion of the carbonate platform, but part oftheir sediments rnay be reworked during a following drawdown. It is important to note that
shallow-water carbonate sedimentation ceases before
basinal evaporite precipitation begins. In the basinal
succession, mudrocks and deep-water carbonates often
pass gradually up into evaporites, and vice versa.
In conclusion, these types of carbonate-evaporite
successions at the basin margin and basin center
should be better referred to as "evaporite-carbonate"
sequences because they begin with evaporites.
they develop more stable shoals and lagoonal
systems.
- Falling sea-Ievel often promotes the influx of
terrigenous material into the basin, particularly
so if the sea-Ievel drops below the edge of a
carbonate platform (mixed carbonatesiliciclastic systems).
- Relatively deep salt basins are characterized by
thick marginal carbonates including some
sabkha and lagoonal evaporites and thick
basinal halite sequences altemating with thin,
partly laminated hernipelagic deep-water sediments and sulfates. These differences have to be
taken into account for the correlation of marginal and basinal evaporite-carbonate sequences.
7.6.1 Incised Valleys on Continental Shelves
and on Land
Relative sea-Ievel fall forces the sea to regress and
rivers to adjust their gradient to the lowered base level.
Because the slope of continental shelves is normally
greater than the gradient of entering rivers of some
size, the rivers cut valleys into pre-existing sediments,
in particular on coastal alluvial plains and inner
shelves (Fig. 7.26a). If relative sea-Ievel fall reaches
about 100 m or more, the entire shelf as well as the
continental slope rnay be affected by valley incision,
delta prograding and slope failures. Additional valleys
beginning at the shelf break and upper slope can develop during the lowstand systems tract, independently
from prolongated land-derived river courses. On land,
valley incision induced by sea-Ievel fall operates
ing late TST and HST causes carbonate accumulation
on both the shelf and basin center (Fig. 7.25f). This
could be more or less the end of the basin filling leaving behind one evaporite-carbonate sequence, but ongoing differential subsidence can more or less restore
the initial basin morphology and thus produce subsequent sequences similar to the first one (Fig. 7.25g).
In this concept, the sequences begin on the shelf
with an unconformity (SB I) on top of carbonates (or
sabkha evaporites ) and in the basin center with a correlative conformity (SB2) on top of hemipelagic thin
marls (± biturninous and larninated). The basinal
evaporites are precipitated in the LST and early TST.
They form thick basinal salt deposits (rnainly halite
and gypsum), a gypsum wedge along the shelf
foreslope, and some sabkha evaporites on the shelf.
7.5.7 Summary (Carbonate Systems, Evaporites)
- Shallow-marine carbonate systems fundamentally deviate from siliciclastic systems due to
their autochthonous, rnainly benthic carbonate
production.
- Vertical growth of carbonate buildups can normally keep up with sea-Ievel rise. Drowning of
carbonate platforrns is mostly associated with
deteriorated environmental conditions for carbonate production or extremely rapid sea-Ievel
nse.
- A surplus in carbonate production, in relation to
sediment accommodation space, leads to
"highstand shedding", the prograding of rimmed
carbonate platforms, and to extensive carbonate
slopes and carbonate "breccias".
- Carbonate ramps respond to sea-Ievel changes in
a similar way as siliciclastic ramp settings, but
7.6 Sequence Stratigraphy of
Transitional Systems
The transitional zone between shallow seas and coastal
lowlands plays a major role in sequence stratigraphy.
It is this zone where relative sea level changes, both
rise and fall, exert the most pronounced influence on
the facies and architecture of sediments. Many of these
transitional environments have already been mentioned
in the facies models of Sects. 7.3 to 7.5 and in Sects.
3.1,3.2 and 3.5. The following seetions focus on (1)
valleys incised during sea-Ievel fall and subsequently
filled with sediment as well as (2) on the repeated
flooding or emergence of coastal alluvial plains, lagoons and lakes.
337
Substantial amounts of shelf carbonates accumulate
during the late TST and early HST when the basin is
flooded. When the water level starts to fall again (late
HST), shallow lagoons and sabkhas can be re-established in the landward portion of the carbonate platform, but part oftheir sediments rnay be reworked during a following drawdown. It is important to note that
shallow-water carbonate sedimentation ceases before
basinal evaporite precipitation begins. In the basinal
succession, mudrocks and deep-water carbonates often
pass gradually up into evaporites, and vice versa.
In conclusion, these types of carbonate-evaporite
successions at the basin margin and basin center
should be better referred to as "evaporite-carbonate"
sequences because they begin with evaporites.
they develop more stable shoals and lagoonal
systems.
- Falling sea-Ievel often promotes the influx of
terrigenous material into the basin, particularly
so if the sea-Ievel drops below the edge of a
carbonate platform (mixed carbonatesiliciclastic systems).
- Relatively deep salt basins are characterized by
thick marginal carbonates including some
sabkha and lagoonal evaporites and thick
basinal halite sequences altemating with thin,
partly laminated hernipelagic deep-water sediments and sulfates. These differences have to be
taken into account for the correlation of marginal and basinal evaporite-carbonate sequences.
7.6.1 Incised Valleys on Continental Shelves
and on Land
Relative sea-Ievel fall forces the sea to regress and
rivers to adjust their gradient to the lowered base level.
Because the slope of continental shelves is normally
greater than the gradient of entering rivers of some
size, the rivers cut valleys into pre-existing sediments,
in particular on coastal alluvial plains and inner
shelves (Fig. 7.26a). If relative sea-Ievel fall reaches
about 100 m or more, the entire shelf as well as the
continental slope rnay be affected by valley incision,
delta prograding and slope failures. Additional valleys
beginning at the shelf break and upper slope can develop during the lowstand systems tract, independently
from prolongated land-derived river courses. On land,
valley incision induced by sea-Ievel fall operates