Whereas the rate of evaporation from open water in
the Persian Gulf is about 124 cm/year, it is only about
6 cm/year in the sabkha. We therefore have only
moderate “evaporite pumping” where chlorides are
precipitated above the water table and sulphates
below the water table. The chlorides deposited will,
however, redissolve as the overburden increases and
the relative position of the water table rises. In some
semi-arid regions, e.g. the Coorong region of
Australia, dolomite lakes form which evaporate to
dryness in the summer (ephemeral lakes). Evaporite
minerals deposited in summer will redissolve during
the winter rains, and will not be preserved in the
bedding series.
6.6
Marine Evaporites
Evaporation from the surface of the sea will cause the
salinity of the water in a basin to increase. If there is
little wave or current action, the warm surface water
will not mix quickly with the underlying, colder water.
If the salt concentration increases, the density will
increase and the surface water will sink to greater
depths and mix with the water there, despite being
warmer. To create a high salinity basin we must have
physical barriers reducing or totally blocking the connection to the open ocean, and the evaporation must be
greater than the total amount of freshwater added to
the basin by precipitation, rivers and groundwater. If
the evaporation is lower than the supply of freshwater
it will develop into a freshwater basin.
An evaporite basin with a limited connection to the
open sea may not dry out; it can then accumulate large
amounts of gypsum, but not more soluble salt like
halite (NaCl). The Mediterranean has enough seawater
exchange through the Straits of Gibraltar to prevent
the formation of evaporites in the Mediterranean
today.
6.7
Tectonic Control of the Formation
of Evaporite Basins
Partial or complete severing of marine basins occurs as
a result of tectonic uplift of barriers. Rifting and incipient spreading of the ocean floor provide ideal
conditions for the formation of evaporite basins, as
marine basins may be partly or entirely cut off by
horsts or by volcanoes and lavas. The Red Sea used
to be an active evaporite basin but gradually the connection with the Indian Ocean has become too large.
Rifting, and spreading of the ocean floor in connection with the formation of the Atlantic Ocean, led to
the accumulation of thick evaporite series in the dry
regions and freshwater lakes in the wetter parts. In
Jurassic and Lower Cretaceous times early seafloor
spreading resulted in the formation of a series of
evaporite basins in the area between Africa and
South America south of the equator, and in the Gulf
of Mexico and North Africa north of the palaeoequator. Where they were sufficiently thick, they
formed diapirs which greatly influenced further sedimentation and the structural development of these
parts of the continental shelf. As seafloor spreading
continued, such that the Atlantic Ocean widened and
the ocean floor basalts cooled and subsided more rapidly, the opportunities for forming closed basins
diminished. After the mid-Cretaceous no major evaporite basins formed in this region. The salt is an ideal
cap rock and both in the Gulf of Mexico and offshore
Brazil large reservoirs of “sub-salt” oil have been
found.
6.8
Evaporites in Lakes and Inland Seas
Basins with no outlet are formed particularly in tectonically active areas. In East Africa we find numerous
freshwater evaporites in the rift basins where the climate is sufficiently arid, and at the end of the
Cainozoic during the Mio-Pliocene there were many
landlocked lakes and inland seas in connection with
rift valleys. Since the chemistry of river water is quite
different from that of seawater, such “continental”
evaporites are quite different from marine series. The
composition will vary according to the types of rocks
and the weathering in the drainage area around the
lake. Lake evaporites normally contain large amounts
of carbonate, particularly sodium carbonate and a
number of other salts, hence the name “soda lakes”.
The mineralogical composition of these soda
deposits is very complex. Two important minerals
are trona Na 2 CO 3 Á NaHCO 3 Á 2H 2 O and gaylussite,
CaCO 3 Á Na 2 CO 3 Á 5H 2 O. If there is volcanism in the
area, as is often the case around rift valleys, this will
modify the composition of the water, both by
6 Mudrocks, Shales, Silica Deposits and Evaporites
225
the Persian Gulf is about 124 cm/year, it is only about
6 cm/year in the sabkha. We therefore have only
moderate “evaporite pumping” where chlorides are
precipitated above the water table and sulphates
below the water table. The chlorides deposited will,
however, redissolve as the overburden increases and
the relative position of the water table rises. In some
semi-arid regions, e.g. the Coorong region of
Australia, dolomite lakes form which evaporate to
dryness in the summer (ephemeral lakes). Evaporite
minerals deposited in summer will redissolve during
the winter rains, and will not be preserved in the
bedding series.
6.6
Marine Evaporites
Evaporation from the surface of the sea will cause the
salinity of the water in a basin to increase. If there is
little wave or current action, the warm surface water
will not mix quickly with the underlying, colder water.
If the salt concentration increases, the density will
increase and the surface water will sink to greater
depths and mix with the water there, despite being
warmer. To create a high salinity basin we must have
physical barriers reducing or totally blocking the connection to the open ocean, and the evaporation must be
greater than the total amount of freshwater added to
the basin by precipitation, rivers and groundwater. If
the evaporation is lower than the supply of freshwater
it will develop into a freshwater basin.
An evaporite basin with a limited connection to the
open sea may not dry out; it can then accumulate large
amounts of gypsum, but not more soluble salt like
halite (NaCl). The Mediterranean has enough seawater
exchange through the Straits of Gibraltar to prevent
the formation of evaporites in the Mediterranean
today.
6.7
Tectonic Control of the Formation
of Evaporite Basins
Partial or complete severing of marine basins occurs as
a result of tectonic uplift of barriers. Rifting and incipient spreading of the ocean floor provide ideal
conditions for the formation of evaporite basins, as
marine basins may be partly or entirely cut off by
horsts or by volcanoes and lavas. The Red Sea used
to be an active evaporite basin but gradually the connection with the Indian Ocean has become too large.
Rifting, and spreading of the ocean floor in connection with the formation of the Atlantic Ocean, led to
the accumulation of thick evaporite series in the dry
regions and freshwater lakes in the wetter parts. In
Jurassic and Lower Cretaceous times early seafloor
spreading resulted in the formation of a series of
evaporite basins in the area between Africa and
South America south of the equator, and in the Gulf
of Mexico and North Africa north of the palaeoequator. Where they were sufficiently thick, they
formed diapirs which greatly influenced further sedimentation and the structural development of these
parts of the continental shelf. As seafloor spreading
continued, such that the Atlantic Ocean widened and
the ocean floor basalts cooled and subsided more rapidly, the opportunities for forming closed basins
diminished. After the mid-Cretaceous no major evaporite basins formed in this region. The salt is an ideal
cap rock and both in the Gulf of Mexico and offshore
Brazil large reservoirs of “sub-salt” oil have been
found.
6.8
Evaporites in Lakes and Inland Seas
Basins with no outlet are formed particularly in tectonically active areas. In East Africa we find numerous
freshwater evaporites in the rift basins where the climate is sufficiently arid, and at the end of the
Cainozoic during the Mio-Pliocene there were many
landlocked lakes and inland seas in connection with
rift valleys. Since the chemistry of river water is quite
different from that of seawater, such “continental”
evaporites are quite different from marine series. The
composition will vary according to the types of rocks
and the weathering in the drainage area around the
lake. Lake evaporites normally contain large amounts
of carbonate, particularly sodium carbonate and a
number of other salts, hence the name “soda lakes”.
The mineralogical composition of these soda
deposits is very complex. Two important minerals
are trona Na 2 CO 3 Á NaHCO 3 Á 2H 2 O and gaylussite,
CaCO 3 Á Na 2 CO 3 Á 5H 2 O. If there is volcanism in the
area, as is often the case around rift valleys, this will
modify the composition of the water, both by
6 Mudrocks, Shales, Silica Deposits and Evaporites
225