6.4 Marine Evaporites
partially, on oceanic crnst. The salts are again covered
by hemipelagic deep-water sediments.
For these and other reasons, most workers believe
that the evaporites below the Mediterranean Sea originate from a closure of the basin in the region of the
present-day Straits of Gibraltar, leading to the desiccation of a formerly water-filled basin of about 1500 m
depth. The connection of the Mediterranean to the
Indian Ocean had been closed during the early Miocene. The "salinity crisis" lasted only for a very short
time interval of 0.2-0.4 Ma; then the basin was again
filled with normal sea water.
Early and late Messinian evaporites observed on
land partially pre-date or post-date the mid-Messinian
desiccation of the deep Mediterranean. These
evaporites accumulated during high sea-Ievel stands in
small shallow basins and coastal sabkhas (Fig. 6.9c).
During the deep desiccation period, rivers entering the
Mediterranean incised deep valleys and canyons to
adjust to the drop of water level, and wider regions
were subjected to increased erosion.
The salts below the Mediterranean were precipitated in shallow water after large drawdown of the sea level. Actually,
even lagoonal and sabkha evaporites have been found in
marginal zones of the evaporites below the sea (cf. Fig.
6.9b), whereas potash salts were discovered in the basin
centers. A drastic change of climate was not necessary for
this development. The huge volunie of salt present below the
sea requires either a high brine concentration prior to the
closure of the Mediterranean via an effective reflux system,
or intensive underground seepage of ocean water into the
shrinking Mediterranean.
High bromium contents in the lower salt unit indicate
evaporation of sea water (see below). According to isotope
studies, the environment at the tope of the upper salt unit
seems to have become brackish due to the influx of river
water.
For further details see, e.g., Hsü et al. (1977), Dronkert
(1985), Busson (1990), McKenzie et al. (1990), Clauson et
al. (1996), Riding et al. (1998).
The Miocene Red Sea evaporites. The generation of
these thick salt deposits is related to those ofthe Mediterranean. They also represent a prominent example of
salt deposition in a deep basin. The salts rest on
thinned continental and, probably due to postdepositional salt flow, locally on young oceanic crnst (cf.
Sect. 4.3 and Fig. 4.7). Whether the salt was deposited
in a deep, brine-filled basin is not clear. Some workers
argue that at least in the upper evaporite section and in
marginal regions of the basin, shallow-water and
sabkha conditions were prevailing.
The Red Sea evaporites were explored by seismic investigations and scientific and commercial deep-sea drilling. Neglecting salt diapirs, they reach a thickness of 1.5-2 km and
a width of 100 km.
During the time of salt deposition, the Red Sea was
open in the north and closed to the Indian Ocean. However,
the connection to the Mediterranean must have been much
271
narrower than the present-day Straits of Bab e\ Mandeb in
the southeast. The present inlet perrnits the exchange ofvery
large water volumes and allows slightly hypersaline conditions only in the northernmost part of the Read Sea.
For further details see, e.g., Stoffers and Kühn (1974),
Kinsman (1975b), Heaton et al. (1995). An exarnple ofsalt
tectonics in the Red Sea is mentioned below (cf. Fig. 6.17df).
Other Ancient Salt Deposits. Large marine salt deposits are known from the late Precambrian throughout
the Phanerozoic, but the maximum of evaporite deposition was obviously reached in the Permian and Triassic, when the supercontinent ofPangea was assembled
and started to break up (cf. Sect. 7.8). The rifting and
early drifting stage of proto-oceans provided, in conjunction with widely extended arid climate, a number
of restrict~d, rapidly subsiding basins favorable for the
deposition of thick evaporite sequences. Further peaks
in salt deposition are known from the early Cambrian
(e.g. on the Siberian Platform), the Devonian, the late
Jurassic to early Cretaceous (e.g. below the Gulf of
Mexico), and the Miocene (Messinian) ofthe Mediterranean. Large salt deposits have been found below the
present passive continental margins and slopes (e.g.,
around the Atlantic Ocean). Other important evaporites
accumulated in marginal and epicontinental basins on
continental or transitional crnst. Examples are listed
and described in the literature mentioned earlier.
6.4.4 Varves and Sedimentation Rates
of Evaporites
One of the most striking phenomena in evaporites is
primary parallellamination (Fig. 6.11a) which occurs
in all types of salt rocks (gypsum and anhydrite, halite,
potash salts) as well as in carbonates associated with
them. The laminae consist of thin couplets of two different sediment types. Well soluble salt layers are separated by much thinner layers of a less soluble salt, for
example halite by anhydrite, or anhydrite by an extremely thin carbonate film. These observations indicate that the laminae are annual varves and originate
from the seasonal climatic change.
The thin carbonate film may be precipitated during the hot
season when the brine is oversaturated with respect to calcium carbonate rather than during the cooler period.
One of the first systematic descriptions of varves in
evaporites has been given by Richter-Bernburg (1960). The
laminated evaporitic facies is most regular and best preserved
in relatively deep parts ofthe basin, but it is also observed in
marginal shallow-water areas. Here the laminae of the same
evaporite facies tend to become thicker than in· the basin
center. This is explained by higher brine concentration, faster
salt precipitation, and intensive early cementation in warmer,
shallower parts of the basin.
The thickness of the laminae (varves) generally depends on the type of evaporite. In the Zechstein salts,
but also in other marine evaporites, the following char-
partially, on oceanic crnst. The salts are again covered
by hemipelagic deep-water sediments.
For these and other reasons, most workers believe
that the evaporites below the Mediterranean Sea originate from a closure of the basin in the region of the
present-day Straits of Gibraltar, leading to the desiccation of a formerly water-filled basin of about 1500 m
depth. The connection of the Mediterranean to the
Indian Ocean had been closed during the early Miocene. The "salinity crisis" lasted only for a very short
time interval of 0.2-0.4 Ma; then the basin was again
filled with normal sea water.
Early and late Messinian evaporites observed on
land partially pre-date or post-date the mid-Messinian
desiccation of the deep Mediterranean. These
evaporites accumulated during high sea-Ievel stands in
small shallow basins and coastal sabkhas (Fig. 6.9c).
During the deep desiccation period, rivers entering the
Mediterranean incised deep valleys and canyons to
adjust to the drop of water level, and wider regions
were subjected to increased erosion.
The salts below the Mediterranean were precipitated in shallow water after large drawdown of the sea level. Actually,
even lagoonal and sabkha evaporites have been found in
marginal zones of the evaporites below the sea (cf. Fig.
6.9b), whereas potash salts were discovered in the basin
centers. A drastic change of climate was not necessary for
this development. The huge volunie of salt present below the
sea requires either a high brine concentration prior to the
closure of the Mediterranean via an effective reflux system,
or intensive underground seepage of ocean water into the
shrinking Mediterranean.
High bromium contents in the lower salt unit indicate
evaporation of sea water (see below). According to isotope
studies, the environment at the tope of the upper salt unit
seems to have become brackish due to the influx of river
water.
For further details see, e.g., Hsü et al. (1977), Dronkert
(1985), Busson (1990), McKenzie et al. (1990), Clauson et
al. (1996), Riding et al. (1998).
The Miocene Red Sea evaporites. The generation of
these thick salt deposits is related to those ofthe Mediterranean. They also represent a prominent example of
salt deposition in a deep basin. The salts rest on
thinned continental and, probably due to postdepositional salt flow, locally on young oceanic crnst (cf.
Sect. 4.3 and Fig. 4.7). Whether the salt was deposited
in a deep, brine-filled basin is not clear. Some workers
argue that at least in the upper evaporite section and in
marginal regions of the basin, shallow-water and
sabkha conditions were prevailing.
The Red Sea evaporites were explored by seismic investigations and scientific and commercial deep-sea drilling. Neglecting salt diapirs, they reach a thickness of 1.5-2 km and
a width of 100 km.
During the time of salt deposition, the Red Sea was
open in the north and closed to the Indian Ocean. However,
the connection to the Mediterranean must have been much
271
narrower than the present-day Straits of Bab e\ Mandeb in
the southeast. The present inlet perrnits the exchange ofvery
large water volumes and allows slightly hypersaline conditions only in the northernmost part of the Read Sea.
For further details see, e.g., Stoffers and Kühn (1974),
Kinsman (1975b), Heaton et al. (1995). An exarnple ofsalt
tectonics in the Red Sea is mentioned below (cf. Fig. 6.17df).
Other Ancient Salt Deposits. Large marine salt deposits are known from the late Precambrian throughout
the Phanerozoic, but the maximum of evaporite deposition was obviously reached in the Permian and Triassic, when the supercontinent ofPangea was assembled
and started to break up (cf. Sect. 7.8). The rifting and
early drifting stage of proto-oceans provided, in conjunction with widely extended arid climate, a number
of restrict~d, rapidly subsiding basins favorable for the
deposition of thick evaporite sequences. Further peaks
in salt deposition are known from the early Cambrian
(e.g. on the Siberian Platform), the Devonian, the late
Jurassic to early Cretaceous (e.g. below the Gulf of
Mexico), and the Miocene (Messinian) ofthe Mediterranean. Large salt deposits have been found below the
present passive continental margins and slopes (e.g.,
around the Atlantic Ocean). Other important evaporites
accumulated in marginal and epicontinental basins on
continental or transitional crnst. Examples are listed
and described in the literature mentioned earlier.
6.4.4 Varves and Sedimentation Rates
of Evaporites
One of the most striking phenomena in evaporites is
primary parallellamination (Fig. 6.11a) which occurs
in all types of salt rocks (gypsum and anhydrite, halite,
potash salts) as well as in carbonates associated with
them. The laminae consist of thin couplets of two different sediment types. Well soluble salt layers are separated by much thinner layers of a less soluble salt, for
example halite by anhydrite, or anhydrite by an extremely thin carbonate film. These observations indicate that the laminae are annual varves and originate
from the seasonal climatic change.
The thin carbonate film may be precipitated during the hot
season when the brine is oversaturated with respect to calcium carbonate rather than during the cooler period.
One of the first systematic descriptions of varves in
evaporites has been given by Richter-Bernburg (1960). The
laminated evaporitic facies is most regular and best preserved
in relatively deep parts ofthe basin, but it is also observed in
marginal shallow-water areas. Here the laminae of the same
evaporite facies tend to become thicker than in· the basin
center. This is explained by higher brine concentration, faster
salt precipitation, and intensive early cementation in warmer,
shallower parts of the basin.
The thickness of the laminae (varves) generally depends on the type of evaporite. In the Zechstein salts,
but also in other marine evaporites, the following char-