6.4 Marine Evaporites
265
SEA LEVEL-CONTROLLED MINOR EVAPORITE CYCLES
a LOW SEA LEVEL
CLASTICS
OPEN SEA
SHELF
BASIN
BLACK SHALE
b HIGH SEA LEVEL
-
-
- - J
4 -
+_
Fig. 6.7. Model for the ongm of minor cyclic
evaporite sequences caused by sea level changes in a
shelf/restricted basin setting. (After Rite 1970, in
Kendall 1988, modified). a No brine reflux during
low and intermediate sea level; algal reef growth on
Relative sea-level changes, regardless of whether
they are caused by eustacy or by tectonic movements,
also generate characteristic transitions from
siliciclastic deposits to evaporites and carbonates. The
carbonates normally represent the high sea-level stand.
Transitions between two or three of these sediment
types occur frequently in the geologic record.
They have been described, for example, from the Persian
Gulf (cf. Sect. 14.3), the northern end of the Red Sea, and
Tunisia (Purser et al. 1987; Ben Ismail and M'Rabet 1990).
Coastal Sabkhas
Evaporite precipitation in coastal sabkhas takes an
intermediate position between primary and secondary
salt formation. The evaporites mainly form below the
land surface, but the driving force is still solar radiation.
-
BLACK SHALE
the shelf barrier occurs coevally with the deposition
of halite and other salts in the basin. b Brine reflux
during high sea level leads to decreasing salinity in
the basin and to a circulation favorable for the accumulation of black shales
Coastal sabkhas represent supratidal flats which are
rarely flooded during spring tides (Sect. 3.2). They
usually develop behind barrier island chains and
hypersaline lagoons (Fig. 6.8a). Their sediments
mainly consist of siliciclastic and biogenic material
transported from tidal and subtidal zones onshore.
Eolian and fluvial material may contribute to the
buildup of the supratidal zone. The surface of such
sabkhas is often controlled by the groundwater table,
which normally gently rises from the beach in a landward direction. Where unconsolidated sandy and silty
sediment above the capillary fringe ofthe groundwater
becomes dry, it may be removed by wind as long as the
sabkha is not overridden by large volumes of dune
sand or fluvial deposits.
The "salt factory" of a coastal sabkha is a mixture
between the scenarios of an inland sabkha (cf. Sect.
2.5) and a marine saIt lagoon. Continental groundwater
from the land may flow seaward and mix with seaderived groundwater. SaIt precipitation is accomplished by evaporitive pumping of groundwater up to
265
SEA LEVEL-CONTROLLED MINOR EVAPORITE CYCLES
a LOW SEA LEVEL
CLASTICS
OPEN SEA
SHELF
BASIN
BLACK SHALE
b HIGH SEA LEVEL
-
-
- - J
4 -
+_
Fig. 6.7. Model for the ongm of minor cyclic
evaporite sequences caused by sea level changes in a
shelf/restricted basin setting. (After Rite 1970, in
Kendall 1988, modified). a No brine reflux during
low and intermediate sea level; algal reef growth on
Relative sea-level changes, regardless of whether
they are caused by eustacy or by tectonic movements,
also generate characteristic transitions from
siliciclastic deposits to evaporites and carbonates. The
carbonates normally represent the high sea-level stand.
Transitions between two or three of these sediment
types occur frequently in the geologic record.
They have been described, for example, from the Persian
Gulf (cf. Sect. 14.3), the northern end of the Red Sea, and
Tunisia (Purser et al. 1987; Ben Ismail and M'Rabet 1990).
Coastal Sabkhas
Evaporite precipitation in coastal sabkhas takes an
intermediate position between primary and secondary
salt formation. The evaporites mainly form below the
land surface, but the driving force is still solar radiation.
-
BLACK SHALE
the shelf barrier occurs coevally with the deposition
of halite and other salts in the basin. b Brine reflux
during high sea level leads to decreasing salinity in
the basin and to a circulation favorable for the accumulation of black shales
Coastal sabkhas represent supratidal flats which are
rarely flooded during spring tides (Sect. 3.2). They
usually develop behind barrier island chains and
hypersaline lagoons (Fig. 6.8a). Their sediments
mainly consist of siliciclastic and biogenic material
transported from tidal and subtidal zones onshore.
Eolian and fluvial material may contribute to the
buildup of the supratidal zone. The surface of such
sabkhas is often controlled by the groundwater table,
which normally gently rises from the beach in a landward direction. Where unconsolidated sandy and silty
sediment above the capillary fringe ofthe groundwater
becomes dry, it may be removed by wind as long as the
sabkha is not overridden by large volumes of dune
sand or fluvial deposits.
The "salt factory" of a coastal sabkha is a mixture
between the scenarios of an inland sabkha (cf. Sect.
2.5) and a marine saIt lagoon. Continental groundwater
from the land may flow seaward and mix with seaderived groundwater. SaIt precipitation is accomplished by evaporitive pumping of groundwater up to