weathering of ash and lava and by the direct addition
of volcanic water (springs) rich in dissolved salts.
In playa lakes there may be total evaporation of
seasonal rainfall resulting in thin layers of carbonates,
including dolomite and sulphates. Signs of dessication
and wind reworking will be common. Salt and clay
particles in the dried-out lakes will tend to be
transported by the wind, to form small dunes. Water
from occasional rains in deserts will often collect
between large aeolian dunes and form interdune
lakes and sabkhas.
6.9
Evaporation of Groundwater
Where groundwater evaporation exceeds rainfall there
is net upward transport to the surface of the soil and
salts have to precipitate out in the soil profile. This can
happen in the vadose zone near the sea, or inland
where evaporation is high and the groundwater has
flowed from another area. Gypsum and chlorides
exert great crystallisation power, and can push aside
the sediment matrix so that large euhedral crystals
form. In brown soil types such as prairie soils, where
the rainfall is high enough to prevent the accumulation
of more soluble salts, a layer of carbonate (caliche) is
commonly formed and in drier areas also gypsum.
When such soils are eroded the broken up caliche
will form pebbly conglomerates.
Evaporites from older geological periods are easily
dissolved by groundwater or surface water. The Dead
Sea is an example of an evaporite basin where water
flowing into the basin is already rich in dissolved salts.
This is because the Jordan river runs through evaporite
sediments of Cretaceous and Tertiary age. In addition,
water of volcanic origin enters the rift valley through
faults and fractures.
6.10 The Stability of Gypsum and
Anhydrite During Burial Diagenesis
Higher pressure and temperature will favour the stability of anhydrite, which has a more compact structure than gypsum. Gypsum formed in evaporite
environments will therefore turn into anhydrite when
there is sufficient overlying sediment, usually at
depths ranging from just under 1,000 to 3,000 m.
When anhydrite-bearing sediments are uplifted and
come into contact with groundwater due to erosion
of overlying sediments they will gradually, depending
on the water circulation, hydrate to gypsum which is
then the stable phase. The loss of water resulting from
the transition from gypsum to anhydrite leads to a
volume reduction (compaction) of 38%, and the transition from anhydrite to gypsum leads to a
corresponding increase (expansion) of 60%. The volume increase can produce near-surface deformation
and faulting in basins where evaporites have been
uplifted due to erosion or diapirism. This compaction
and expansion creates serious geotechnical problems
in those areas of Europe, e.g. Switzerland and
Germany, where evaporite deposits are common.
Sulphates are rather stable and are not easily reduced
to sulphides inorganically; this requires sulphatereducing bacteria. However in evaporite deposits
which have been buried to depths where the temperature has exceeded 80
C, sulphate-reducing bacteria
have not survived and the rocks are then pasteurised.
The reduction of sulphides like gypsum and anhydrite
will then be very slow even at shallow depth.
6.11 Iron- and Manganese-Rich
Sediments
Iron and manganese share many similarities in their
geochemical behaviour in sedimentary environments.
Both elements are poorly soluble in the oxidised state
because they form hydroxides and oxides: Fe(OH) 3 ,
Mn(OH) 4 , Fe 2 O 3 and MnO 2 . In the reduced state they
occur as Fe
2+ and Mn
2+ and are then much more
soluble. Both iron and manganese can be precipitated
as carbonate (FeCO 3 , MnCO 3 ), either as separate
minerals, or as part of calcite, dolomite or ankerite.
They are therefore not very soluble in basic solutions
with low redox potentials, but are quite soluble at low
pH values and in the reduced state. Consequently we
can precipitate iron and manganese in two ways: (1)
through oxidation, (2) by changing the solution from
acid to basic.
Fe
2+ forms iron sulphides with very low solubility
while manganese sulphides are far more soluble and
therefore more soluble in porewater which is reducing
and acid. Based on these geochemical considerations,
we can predict a great deal about the deposition of iron
and manganese in sediments. As silicates and other
minerals dissolve in the oxidising environment during
226
K. Bjørlykke
of volcanic water (springs) rich in dissolved salts.
In playa lakes there may be total evaporation of
seasonal rainfall resulting in thin layers of carbonates,
including dolomite and sulphates. Signs of dessication
and wind reworking will be common. Salt and clay
particles in the dried-out lakes will tend to be
transported by the wind, to form small dunes. Water
from occasional rains in deserts will often collect
between large aeolian dunes and form interdune
lakes and sabkhas.
6.9
Evaporation of Groundwater
Where groundwater evaporation exceeds rainfall there
is net upward transport to the surface of the soil and
salts have to precipitate out in the soil profile. This can
happen in the vadose zone near the sea, or inland
where evaporation is high and the groundwater has
flowed from another area. Gypsum and chlorides
exert great crystallisation power, and can push aside
the sediment matrix so that large euhedral crystals
form. In brown soil types such as prairie soils, where
the rainfall is high enough to prevent the accumulation
of more soluble salts, a layer of carbonate (caliche) is
commonly formed and in drier areas also gypsum.
When such soils are eroded the broken up caliche
will form pebbly conglomerates.
Evaporites from older geological periods are easily
dissolved by groundwater or surface water. The Dead
Sea is an example of an evaporite basin where water
flowing into the basin is already rich in dissolved salts.
This is because the Jordan river runs through evaporite
sediments of Cretaceous and Tertiary age. In addition,
water of volcanic origin enters the rift valley through
faults and fractures.
6.10 The Stability of Gypsum and
Anhydrite During Burial Diagenesis
Higher pressure and temperature will favour the stability of anhydrite, which has a more compact structure than gypsum. Gypsum formed in evaporite
environments will therefore turn into anhydrite when
there is sufficient overlying sediment, usually at
depths ranging from just under 1,000 to 3,000 m.
When anhydrite-bearing sediments are uplifted and
come into contact with groundwater due to erosion
of overlying sediments they will gradually, depending
on the water circulation, hydrate to gypsum which is
then the stable phase. The loss of water resulting from
the transition from gypsum to anhydrite leads to a
volume reduction (compaction) of 38%, and the transition from anhydrite to gypsum leads to a
corresponding increase (expansion) of 60%. The volume increase can produce near-surface deformation
and faulting in basins where evaporites have been
uplifted due to erosion or diapirism. This compaction
and expansion creates serious geotechnical problems
in those areas of Europe, e.g. Switzerland and
Germany, where evaporite deposits are common.
Sulphates are rather stable and are not easily reduced
to sulphides inorganically; this requires sulphatereducing bacteria. However in evaporite deposits
which have been buried to depths where the temperature has exceeded 80
C, sulphate-reducing bacteria
have not survived and the rocks are then pasteurised.
The reduction of sulphides like gypsum and anhydrite
will then be very slow even at shallow depth.
6.11 Iron- and Manganese-Rich
Sediments
Iron and manganese share many similarities in their
geochemical behaviour in sedimentary environments.
Both elements are poorly soluble in the oxidised state
because they form hydroxides and oxides: Fe(OH) 3 ,
Mn(OH) 4 , Fe 2 O 3 and MnO 2 . In the reduced state they
occur as Fe
2+ and Mn
2+ and are then much more
soluble. Both iron and manganese can be precipitated
as carbonate (FeCO 3 , MnCO 3 ), either as separate
minerals, or as part of calcite, dolomite or ankerite.
They are therefore not very soluble in basic solutions
with low redox potentials, but are quite soluble at low
pH values and in the reduced state. Consequently we
can precipitate iron and manganese in two ways: (1)
through oxidation, (2) by changing the solution from
acid to basic.
Fe
2+ forms iron sulphides with very low solubility
while manganese sulphides are far more soluble and
therefore more soluble in porewater which is reducing
and acid. Based on these geochemical considerations,
we can predict a great deal about the deposition of iron
and manganese in sediments. As silicates and other
minerals dissolve in the oxidising environment during
226
K. Bjørlykke