6.4 Marine Evaporites
Brine mayaiso be lost to the underground where it
drives away less dense groundwater. Such leaky basins
normally lose their best soluble salts and therefore rarely
preeipitate and preserve potash or bittern salts.
Thesynsedimentarysubsidence model (Fig. 6.5a). This
model was proposed to explain the formation of salt
deposits thieker than the limited depth of a silled lagoon.
Normal slow tectonic subsidenee is enhaneed by the
isostatie effeet ofthe rapidly aeeumulating salt deposits
on top of (usually) eontinental erust (cf. Seet. 8.1). In
addition, differential subsidenee within the basin may
eause lateral variations in the evaporite thickness. Ifthe
basin is filled up, the residual brine precipitating K and
Mg salts may be eolleeted in ponds above the areas of
maximum subsidence.
However, the great thicknesses of individual cyc1es of salt
deposits frequently observed cannot be interpreted adequately
by this mechanism, except for rapidly subsiding rift zones and
strike-slip basins (Sect. 1.2). If the shallow basin is rapidly
filled up by one evaporative phase, the time for the ernst to
react to the applied load is too short to cause substantial additional subsidence.
Thick salt deposits can be deposited only if a subsiding
basin experiences several evaporative cycles interrupted by
long intervals of starved sedimentation or non-deposition.
During these intervals, continued tectonic and salt load-driven
subsidence can create sufficient space for further significant
salt accumulation (see below).
Drawdown ofWater Level in Deep Basin
A number of salt deposits, whieh most likely formed in
deep basins, clearly show evidenee for shallow water
and even sabkha environments. These occurrences are
explained by two related processes: (1) evaporative
drawdown of the water level after the basinal brine had
already reached a relatively high eoneentration, and (2)
closing of the barrier to the oeean. Seepage below the
barrier or episodie spill-overs may deliver further sea
water, besides some fresh water, into the shrinking
basin (cf. Seet. 7.5.6, Fig. 7.25). Saturation for the
different evaporites is established in a similar way as
deseribed for the closed sea-water model. In order to
initiate halite preeipitation, the water level must fall
deep below the original level, and the final stages of
evaporite preeipitation oeeur under playa and sabkha
conditions. The areal distribution of the resulting salt
deposits therefore resembles a bull's eye pattern (Fig.
6.4a).
Deep-Water Salt Basin
This model (Fig. 6.5b,c) is another attempt to explain
a rapidly deposited thick evaporite sequence without
synsedimentary subsidenee (Schmalz 1969). The deep
basin represents an adjaeent basin on thinned eontinen263
tal or oeeanic erust close to the open oeean. The prineipal points of this model are briefly summarized as
follows:
- Stage 1. The gateway to the open sea is narrowing.
As a result, the former deep-reaching water circulation
is replaeed by a eireulation system restrieted to the
upper water layer (Fig. 6.5b). In this stage, the lower
water body has already reached a higher density (1.07
to 1.08 g/cm 3 ) than the surface water, where the production of phytoplankton continues. Normal
hemipelagie and biogenic marine sediments are then
followed by bituminous marls or limestones
(Stinkkalke) deposited in stagnant suboxic or anoxie
deep water.
- Stage 2. In the sueceeding stage, caused by further
narrowing or shallowing of the sill, the surfaee brine
increases in density and starts to sink on the landward
side of the basin. At the beginning of this proeess,
gypsum and halite preeipitate at the water surface and
are re-dissolved when they sink into less concentrated
deeper water.
- Stage 3. The subsequent development of the basin is
controversially diseussed. Aeeording to Schmalz
(1969), halite, gypsum, and some carbonates are deposited rapidly and more or less simultaneously, generating a lateral facies suecession as shown in Fig. 6.5c.
Onee the basin is partially or nearly filled up, residual
brines oceupy smaller ponds on top of halite.
Field evidence from many large evaporite basins, including the European Zechstein basin (see below), and
the sequence stratigraphie coneept for such settings
(cf. Sect. 7.5.6) do not sufficiently support this model.
Normally, carbonates and sulfates aecumulate along
the basin margin and on topographie highs within the
basin, before halite is precipitated in the more eentral
parts of the basin. It was also pointed out (Kendall
(1988) that the brine concentration of a deep, silled
basin must be more or less homogeneous laterally,
beeause differences in water density can hardly be
maintained in such a basin. Therefore, the deep-water
basin appears to be incapable of simultaneously precipitating different mineral facies in deep water.
A modification of this model is a mixed-source
deep salt basin (Anderson and Dean 1995). This basin
may be reeharged by both sea-water seepage and meteoric water (mostly groundwater, Fig. 6.6a). A marine
reflux system is not necessary because both brine concentration and a certain hydrologic balance between
inflow and water loss is accomplished by evaporation.
A small volume of marine inflow relative to meteorie
water recharge is sufficient to aeeount for the overall
distribution ofhalite and sulfate in the basin. However,
the water level of the basin fluctuates in a similar way
as that of closed lake basin. During high water level,
carbonate (± bituminous) and gypsum (anhydrite) are
precipitated; during low level, the seepage of sea water
Brine mayaiso be lost to the underground where it
drives away less dense groundwater. Such leaky basins
normally lose their best soluble salts and therefore rarely
preeipitate and preserve potash or bittern salts.
Thesynsedimentarysubsidence model (Fig. 6.5a). This
model was proposed to explain the formation of salt
deposits thieker than the limited depth of a silled lagoon.
Normal slow tectonic subsidenee is enhaneed by the
isostatie effeet ofthe rapidly aeeumulating salt deposits
on top of (usually) eontinental erust (cf. Seet. 8.1). In
addition, differential subsidenee within the basin may
eause lateral variations in the evaporite thickness. Ifthe
basin is filled up, the residual brine precipitating K and
Mg salts may be eolleeted in ponds above the areas of
maximum subsidence.
However, the great thicknesses of individual cyc1es of salt
deposits frequently observed cannot be interpreted adequately
by this mechanism, except for rapidly subsiding rift zones and
strike-slip basins (Sect. 1.2). If the shallow basin is rapidly
filled up by one evaporative phase, the time for the ernst to
react to the applied load is too short to cause substantial additional subsidence.
Thick salt deposits can be deposited only if a subsiding
basin experiences several evaporative cycles interrupted by
long intervals of starved sedimentation or non-deposition.
During these intervals, continued tectonic and salt load-driven
subsidence can create sufficient space for further significant
salt accumulation (see below).
Drawdown ofWater Level in Deep Basin
A number of salt deposits, whieh most likely formed in
deep basins, clearly show evidenee for shallow water
and even sabkha environments. These occurrences are
explained by two related processes: (1) evaporative
drawdown of the water level after the basinal brine had
already reached a relatively high eoneentration, and (2)
closing of the barrier to the oeean. Seepage below the
barrier or episodie spill-overs may deliver further sea
water, besides some fresh water, into the shrinking
basin (cf. Seet. 7.5.6, Fig. 7.25). Saturation for the
different evaporites is established in a similar way as
deseribed for the closed sea-water model. In order to
initiate halite preeipitation, the water level must fall
deep below the original level, and the final stages of
evaporite preeipitation oeeur under playa and sabkha
conditions. The areal distribution of the resulting salt
deposits therefore resembles a bull's eye pattern (Fig.
6.4a).
Deep-Water Salt Basin
This model (Fig. 6.5b,c) is another attempt to explain
a rapidly deposited thick evaporite sequence without
synsedimentary subsidenee (Schmalz 1969). The deep
basin represents an adjaeent basin on thinned eontinen263
tal or oeeanic erust close to the open oeean. The prineipal points of this model are briefly summarized as
follows:
- Stage 1. The gateway to the open sea is narrowing.
As a result, the former deep-reaching water circulation
is replaeed by a eireulation system restrieted to the
upper water layer (Fig. 6.5b). In this stage, the lower
water body has already reached a higher density (1.07
to 1.08 g/cm 3 ) than the surface water, where the production of phytoplankton continues. Normal
hemipelagie and biogenic marine sediments are then
followed by bituminous marls or limestones
(Stinkkalke) deposited in stagnant suboxic or anoxie
deep water.
- Stage 2. In the sueceeding stage, caused by further
narrowing or shallowing of the sill, the surfaee brine
increases in density and starts to sink on the landward
side of the basin. At the beginning of this proeess,
gypsum and halite preeipitate at the water surface and
are re-dissolved when they sink into less concentrated
deeper water.
- Stage 3. The subsequent development of the basin is
controversially diseussed. Aeeording to Schmalz
(1969), halite, gypsum, and some carbonates are deposited rapidly and more or less simultaneously, generating a lateral facies suecession as shown in Fig. 6.5c.
Onee the basin is partially or nearly filled up, residual
brines oceupy smaller ponds on top of halite.
Field evidence from many large evaporite basins, including the European Zechstein basin (see below), and
the sequence stratigraphie coneept for such settings
(cf. Sect. 7.5.6) do not sufficiently support this model.
Normally, carbonates and sulfates aecumulate along
the basin margin and on topographie highs within the
basin, before halite is precipitated in the more eentral
parts of the basin. It was also pointed out (Kendall
(1988) that the brine concentration of a deep, silled
basin must be more or less homogeneous laterally,
beeause differences in water density can hardly be
maintained in such a basin. Therefore, the deep-water
basin appears to be incapable of simultaneously precipitating different mineral facies in deep water.
A modification of this model is a mixed-source
deep salt basin (Anderson and Dean 1995). This basin
may be reeharged by both sea-water seepage and meteoric water (mostly groundwater, Fig. 6.6a). A marine
reflux system is not necessary because both brine concentration and a certain hydrologic balance between
inflow and water loss is accomplished by evaporation.
A small volume of marine inflow relative to meteorie
water recharge is sufficient to aeeount for the overall
distribution ofhalite and sulfate in the basin. However,
the water level of the basin fluctuates in a similar way
as that of closed lake basin. During high water level,
carbonate (± bituminous) and gypsum (anhydrite) are
precipitated; during low level, the seepage of sea water