274
mally show sequences different from the basin centers
(e.g. the anhydrite section ofFig. 6.11b; cf. Sect. 7.5.6
and Fig. 7.25).
Sedimentary structures of evaporites, inc1uding specitic crystal shapes, are either of primary origin (such
as the varves) or predominantly generated by
diagenetic processes. In brine pools and along their
margins, the salt minerals basically form in three different ways:
- Crystal growth at the water surface or at the
pycnoc1ine (haloc1ine) between lower mineralized
surface water and the deeper brine. When they have
reached some size, the crystals settle without significant further change.
- Crystal growth predominantly at the sediment-water
interface (bottom-nuc1eated crystals). The size ofthese
crystals tends to be large and the porosity of the fresh
salt deposit is limited.
- Crystal growth within the sediment (displacive crystals). These may be synsedimentary (sabkha evaporites) or of diagenetic origin.
Primary salts are often affected by brine dilution creating microkarst phenomena or a kind of ablation surface
at the top of the salt deposit. Repeated near-surface
dissolution and reprecipitation of salts can disturb
primary beds or lamination (haloturbation) and create
"chaotic halite" (Fig. 6.l2b). In addition, primary salt
crystals can be transformed to other types of salt minerals by so-called back-reactions with the parent brine
which has changed its composition since the formation
of the primary minerals. Back-reactions can occur at
the sediment surface and in the subsurface. The resulting pseudomorphs. e.g. anhydrite after gypsum or glauberite and polyhalite after anhydrite, often display the
crystal form of the preceeding mineral phase. Under
favorable conditions, the special features of the primary crystals are preserved as pseudomorphs (Fig.
6.l2b).
Furthermore, bottom-nucleated gypsum and halite
can be reworked in the shaIlow-water zone and display
cross-bedding and ripple marks. Primary crystals may
be overgrown and partially form pisolitic grains.
Evaporites of comparatively deep basins often contain
redeposited carbonates and salts from shallow water in
the form of mass flows and turbidites (Fig. 6.l2b; cf.
Sect. 5.4). These features are, however, frequently
difficult to identify and to distinguish from diagenetic
structures (see below).
6.4.6 Diagenesis of Evaporites
(Secondary and Tertiary Evaporites)
After burial, the primary salt deposits, particularly
those precipitated under a standing water body, are
strongly affected by mechanical and physicochemicalChapter 6 Special Depositional Environments
mineralogical diagenesis. One can distinguish the following principal regimes of fluid flow which in turn
are associated with a number of additional processes
(Fig. 6.12a):
- Active phreatic flow of meteroic water or brine
within and below the salt deposit.
- Compaction flow driven by the expulsion of porewater from the sediment.
- Thermobaric flow fed by dehydration of evaporite
minerals and c1ay minerals and driven by differences in pressure head.
- Pore fluids in the zone of metamorphism
(T>200°C).
The associated processes generating new and destroying or replacing primary salt and other minerals include:
- Early and late cementation.
- Replacement of pre-existing minerals.
- Subsurface dissolution.
Change of mineral phases and release of water.
Some of these processes are briefly discussed here.
The Active Phreatic Zone
Influx of meteoric water, sea water, or brine into the
edges of an evaporite basin and sinking of brine from
the salt pool (brine reflux) can create fluids ofvarious
chemical composition circulating within and below the
salt deposits (Fig. 6.12a). The interplay ofthese fluids
can be complex in relation to the changing rock permeability in the subsurface. The buried salts become increasingly impt:rmeable (e.g. halite at one to a few
hundred meters ofburial depth) and thus tend to separate shallow and deeper flow paths. In this flow regime, both dissolution of primary minerals and replacement of pre-existing minerals as weIl as the formation of new minerals take place. Below the central
parts of the basin where highly concentrated brines
sink into the subsurface, for example halite pseudomorphs may replace primary bottom-nucleated gypsum
(Fig. 6.12b), or pre-evaporitic limestone can be replaced by secondary dolomite. At outlet seeps ofbrine,
spring mounds can form.
Mechanical Compaction and Compaction Flow
Mechanical compaction of evaporites is substantial
and can therefore bring about great differences in the
thicknesses between the primary and secondary salt
deposits, as weIl as between different types of
evaporites. Particularly in areas where fine-grained
crystals have settled through the water column, the
mally show sequences different from the basin centers
(e.g. the anhydrite section ofFig. 6.11b; cf. Sect. 7.5.6
and Fig. 7.25).
Sedimentary structures of evaporites, inc1uding specitic crystal shapes, are either of primary origin (such
as the varves) or predominantly generated by
diagenetic processes. In brine pools and along their
margins, the salt minerals basically form in three different ways:
- Crystal growth at the water surface or at the
pycnoc1ine (haloc1ine) between lower mineralized
surface water and the deeper brine. When they have
reached some size, the crystals settle without significant further change.
- Crystal growth predominantly at the sediment-water
interface (bottom-nuc1eated crystals). The size ofthese
crystals tends to be large and the porosity of the fresh
salt deposit is limited.
- Crystal growth within the sediment (displacive crystals). These may be synsedimentary (sabkha evaporites) or of diagenetic origin.
Primary salts are often affected by brine dilution creating microkarst phenomena or a kind of ablation surface
at the top of the salt deposit. Repeated near-surface
dissolution and reprecipitation of salts can disturb
primary beds or lamination (haloturbation) and create
"chaotic halite" (Fig. 6.l2b). In addition, primary salt
crystals can be transformed to other types of salt minerals by so-called back-reactions with the parent brine
which has changed its composition since the formation
of the primary minerals. Back-reactions can occur at
the sediment surface and in the subsurface. The resulting pseudomorphs. e.g. anhydrite after gypsum or glauberite and polyhalite after anhydrite, often display the
crystal form of the preceeding mineral phase. Under
favorable conditions, the special features of the primary crystals are preserved as pseudomorphs (Fig.
6.l2b).
Furthermore, bottom-nucleated gypsum and halite
can be reworked in the shaIlow-water zone and display
cross-bedding and ripple marks. Primary crystals may
be overgrown and partially form pisolitic grains.
Evaporites of comparatively deep basins often contain
redeposited carbonates and salts from shallow water in
the form of mass flows and turbidites (Fig. 6.l2b; cf.
Sect. 5.4). These features are, however, frequently
difficult to identify and to distinguish from diagenetic
structures (see below).
6.4.6 Diagenesis of Evaporites
(Secondary and Tertiary Evaporites)
After burial, the primary salt deposits, particularly
those precipitated under a standing water body, are
strongly affected by mechanical and physicochemicalChapter 6 Special Depositional Environments
mineralogical diagenesis. One can distinguish the following principal regimes of fluid flow which in turn
are associated with a number of additional processes
(Fig. 6.12a):
- Active phreatic flow of meteroic water or brine
within and below the salt deposit.
- Compaction flow driven by the expulsion of porewater from the sediment.
- Thermobaric flow fed by dehydration of evaporite
minerals and c1ay minerals and driven by differences in pressure head.
- Pore fluids in the zone of metamorphism
(T>200°C).
The associated processes generating new and destroying or replacing primary salt and other minerals include:
- Early and late cementation.
- Replacement of pre-existing minerals.
- Subsurface dissolution.
Change of mineral phases and release of water.
Some of these processes are briefly discussed here.
The Active Phreatic Zone
Influx of meteoric water, sea water, or brine into the
edges of an evaporite basin and sinking of brine from
the salt pool (brine reflux) can create fluids ofvarious
chemical composition circulating within and below the
salt deposits (Fig. 6.12a). The interplay ofthese fluids
can be complex in relation to the changing rock permeability in the subsurface. The buried salts become increasingly impt:rmeable (e.g. halite at one to a few
hundred meters ofburial depth) and thus tend to separate shallow and deeper flow paths. In this flow regime, both dissolution of primary minerals and replacement of pre-existing minerals as weIl as the formation of new minerals take place. Below the central
parts of the basin where highly concentrated brines
sink into the subsurface, for example halite pseudomorphs may replace primary bottom-nucleated gypsum
(Fig. 6.12b), or pre-evaporitic limestone can be replaced by secondary dolomite. At outlet seeps ofbrine,
spring mounds can form.
Mechanical Compaction and Compaction Flow
Mechanical compaction of evaporites is substantial
and can therefore bring about great differences in the
thicknesses between the primary and secondary salt
deposits, as weIl as between different types of
evaporites. Particularly in areas where fine-grained
crystals have settled through the water column, the