6.4 Marine Evaporites
the sabkha surface. Where the concentration of the
brine exceeds saturation for calcium sulfate and possibly also for halite, gypsum and halite form displacive
nodules, lenses, or crusts. The minerals precipitated in
the different zones of the intertidal and supratidal area
are shown in Fig. 6.8b.
The precipitation of gypsum begins in the lower sabkha
within the algal mats. Primary anhydrite can fonn in the
higher landward part ofthe sabkha, where the brine concentration approaches saturation for halite. Ralite is often redissolved during floods which also deliver new salts into the
subsurface brine system. LocaUy, more halite and even potash salts may accumulate in smaU ponds eroded by earlier
floods.
It is assumed that a kind ofbrine reflux takes place underground, possibly along older buried valley or channel fiUs.
Provided the concentration ofthe brine becomes sufficiently
high in the pore system, gypsum or anhydrite are also generated below the groundwater table in the phreatic zone, particularly in the upper supratidal flats (Fig. 6.8b-d).
Primary gypsum may be overgrown later by anhydrite
or transformed into anhydrite, which often shows characteristic contorted beds, small diapiric structures, or a
"chicken-wire" pattem(Fig. 6.8d). Underthe influence
of increasing Mg/Ca ratios, primary aragonite and
calcite are partially or entirely converted to dolomite.
Because the host sediment of the supratidal zone is
often poor in organic matter (due to oxidation), the
sabkha beds tend to be primarily brown. Later, after
diagenesis, they usually become red in color.
The upper limit of one cycle of sabkha evaporites is
controlled by the capillary fring(! of the groundwater
table. When this fringe can no longer reach the surface
and is overlain by dry sandy material, the evaporation
of groundwater practicallY ceases. Consequently, the
sabkha evaporites are usually overlain either by
windblown sand or fluvial deposits (Fig. 6.8d). The
lower boundary of the sabkha evaporites depends on
the concentration of brine reached below the water
table. Thus, assuming constant sea level and neglecting
subsidence, the thickness of such a cycle is approximately one to a few meters, and its areal extent is limited.
However, under conditions of changing sea level
and subsidence, sabkha evaporites can reach greater
thicknesses and form cyclic sequences (Fig. 6.8e).
Channels, storm erosion, and redeposition of sabkha
sediments in the intertidal or subtidal zone can complicate such a simple cyclicity model. In addition, relative
sea-Ievel changes can lead to wider extended sabkha
evaporites than those observed in present-day exampIes (see, e.g., Purser 1985). Due to frequent sea levelchanges in the past (cf. Chap. 7), this type of
evaporites is very common in the ancient record.
The conceptual model ofFigure 6.8 can be modified in relation to the local paleogeographic situation. The most important alternative, not shown in the figure, is the transition from
267
a salt lagoon to a sabkha. In this case, lagoonal evaporites
formed adjacent to coastal sabkhas rnay alternate with sabkha
evaporites. This development is frequently observed in ancient sediments.
The Combined Deep and ShaUow/Sabkha
Salt Basin
General Aspects
The basic salt basin models in their pure form cannot
explain all the phenomena observed in ancient regions
of salt deposition. Especially the origin ofthe "saline
giants" has been and still is the object of lively debates.
See, e.g., Rsü (1972), Jauzein (1984), Sonnenfeld and
Perthuisot (1989), Bussot (1990).
One of the reasons for this uncertainty is the lack of
present-day examp1es for such enonnous1y thick (up to severa1 thousunds of meters) and widely extended evaporites
(several thousands ofkilometers long and hundreds ofkilometers wide) as known from Precambrian to Miocene times.
No reallarge or deep salt basin exists in the modern world,
probably because we are living in an interglacial period. The
modern temperature gradients between the equator and the
poles are unusually high, and the continents are widely distributed over the surface of the globe. This situation is not
favorable for the generation oflarge evaporite deposits.
To explain some ofthe 'ancient giant salt deposits,
two or three of the simple salt basin models have to be
cornbined. Furthermore, substantial relative sea-Ievel
changes have to be taken into account.
Evolution of a Complex Salt Basin
The evolution of carbonate-evaporite systems in the
context of sequence stratigraphy is described in Sect.
7.5.6 and Fig. 7.25. Here, a brief summary is given, in
which the evolution begins with an inland basin. The
succeeding large and complex salt basin may show the
following stages of development (Fig. 6.9):
(1) Desert drainage basin. A continuously subsiding
large inland basin on continental or transitional crust is
separated from the open sea by aland baITier. It receives, due to arid climate and low relief in its drainage area, little sediment. After some time, the basin
floor drops below mean sea level.
(2) Flooding ofthe basin by the sea. During a phase of
particularly high sea level, the entire basin is flooded
(Fig. 6.9a). As long as the water exchange with the
open ocean is sufficiently great, normal-marine
hemipelagic sediments are deposited in wide areas of
the basin. Along the basin margin, shelf carbonates,
lagoonal and sabkha evaporites can accumulate. Shelf
edges, submarine ridges and highs are preferential sites
of algal reef growth in warm waters.
the sabkha surface. Where the concentration of the
brine exceeds saturation for calcium sulfate and possibly also for halite, gypsum and halite form displacive
nodules, lenses, or crusts. The minerals precipitated in
the different zones of the intertidal and supratidal area
are shown in Fig. 6.8b.
The precipitation of gypsum begins in the lower sabkha
within the algal mats. Primary anhydrite can fonn in the
higher landward part ofthe sabkha, where the brine concentration approaches saturation for halite. Ralite is often redissolved during floods which also deliver new salts into the
subsurface brine system. LocaUy, more halite and even potash salts may accumulate in smaU ponds eroded by earlier
floods.
It is assumed that a kind ofbrine reflux takes place underground, possibly along older buried valley or channel fiUs.
Provided the concentration ofthe brine becomes sufficiently
high in the pore system, gypsum or anhydrite are also generated below the groundwater table in the phreatic zone, particularly in the upper supratidal flats (Fig. 6.8b-d).
Primary gypsum may be overgrown later by anhydrite
or transformed into anhydrite, which often shows characteristic contorted beds, small diapiric structures, or a
"chicken-wire" pattem(Fig. 6.8d). Underthe influence
of increasing Mg/Ca ratios, primary aragonite and
calcite are partially or entirely converted to dolomite.
Because the host sediment of the supratidal zone is
often poor in organic matter (due to oxidation), the
sabkha beds tend to be primarily brown. Later, after
diagenesis, they usually become red in color.
The upper limit of one cycle of sabkha evaporites is
controlled by the capillary fring(! of the groundwater
table. When this fringe can no longer reach the surface
and is overlain by dry sandy material, the evaporation
of groundwater practicallY ceases. Consequently, the
sabkha evaporites are usually overlain either by
windblown sand or fluvial deposits (Fig. 6.8d). The
lower boundary of the sabkha evaporites depends on
the concentration of brine reached below the water
table. Thus, assuming constant sea level and neglecting
subsidence, the thickness of such a cycle is approximately one to a few meters, and its areal extent is limited.
However, under conditions of changing sea level
and subsidence, sabkha evaporites can reach greater
thicknesses and form cyclic sequences (Fig. 6.8e).
Channels, storm erosion, and redeposition of sabkha
sediments in the intertidal or subtidal zone can complicate such a simple cyclicity model. In addition, relative
sea-Ievel changes can lead to wider extended sabkha
evaporites than those observed in present-day exampIes (see, e.g., Purser 1985). Due to frequent sea levelchanges in the past (cf. Chap. 7), this type of
evaporites is very common in the ancient record.
The conceptual model ofFigure 6.8 can be modified in relation to the local paleogeographic situation. The most important alternative, not shown in the figure, is the transition from
267
a salt lagoon to a sabkha. In this case, lagoonal evaporites
formed adjacent to coastal sabkhas rnay alternate with sabkha
evaporites. This development is frequently observed in ancient sediments.
The Combined Deep and ShaUow/Sabkha
Salt Basin
General Aspects
The basic salt basin models in their pure form cannot
explain all the phenomena observed in ancient regions
of salt deposition. Especially the origin ofthe "saline
giants" has been and still is the object of lively debates.
See, e.g., Rsü (1972), Jauzein (1984), Sonnenfeld and
Perthuisot (1989), Bussot (1990).
One of the reasons for this uncertainty is the lack of
present-day examp1es for such enonnous1y thick (up to severa1 thousunds of meters) and widely extended evaporites
(several thousands ofkilometers long and hundreds ofkilometers wide) as known from Precambrian to Miocene times.
No reallarge or deep salt basin exists in the modern world,
probably because we are living in an interglacial period. The
modern temperature gradients between the equator and the
poles are unusually high, and the continents are widely distributed over the surface of the globe. This situation is not
favorable for the generation oflarge evaporite deposits.
To explain some ofthe 'ancient giant salt deposits,
two or three of the simple salt basin models have to be
cornbined. Furthermore, substantial relative sea-Ievel
changes have to be taken into account.
Evolution of a Complex Salt Basin
The evolution of carbonate-evaporite systems in the
context of sequence stratigraphy is described in Sect.
7.5.6 and Fig. 7.25. Here, a brief summary is given, in
which the evolution begins with an inland basin. The
succeeding large and complex salt basin may show the
following stages of development (Fig. 6.9):
(1) Desert drainage basin. A continuously subsiding
large inland basin on continental or transitional crust is
separated from the open sea by aland baITier. It receives, due to arid climate and low relief in its drainage area, little sediment. After some time, the basin
floor drops below mean sea level.
(2) Flooding ofthe basin by the sea. During a phase of
particularly high sea level, the entire basin is flooded
(Fig. 6.9a). As long as the water exchange with the
open ocean is sufficiently great, normal-marine
hemipelagic sediments are deposited in wide areas of
the basin. Along the basin margin, shelf carbonates,
lagoonal and sabkha evaporites can accumulate. Shelf
edges, submarine ridges and highs are preferential sites
of algal reef growth in warm waters.