276
porosity of the primary sediment is high. Differential
compaction between early cemented marginal and later
cemented central evaporites may somewhat accentuate
differences in bathymetry ofthe depositional basin and
therefore mislead reconstructions of the original basin
topography (cf. Fig. 6.11d). The differential eompaction of early, but only partially eemented calcium sulfate deposits may be one of the processes eausing collapse breccias (Fig. 6.11e). More commonare breecias
resulting from mass flows from topographie highs into
deeper parts of the evaporite basin. Sedimentary breccias also originate from the reworking of salt crusts on
tidal flats.
Sediment compaction expulses pore water out of
both the evaporites and other sediments. As long as the
overlying material is sufficiently permeable, the compaction flow is directed upward. However, salts such
as halite lose their porosity already at shallow burial
depth. Halite crystals generate a kind of mosaic texture
in which the grain boundaries are eurved and display
sutures (dissolution seams). When the overlying salts
act as aseal, the fluids flow laterally updip (Fig.
6.12a).
Along their paths the fluids ean dissolve pre-existing salt minerals, alter older sediments, and preeipitate
new minerals as eement. Dissolution leaves behind
various residues such as evaporite dissolution breccias,
rauhwacken, silicified and calcitized evaporite nodules. Reaction of the fluids with roeks ean cause, for
example, dolomitization of calcarous material or, in
some eases, calcitization of already dolomitized strata.
Cementation by salt minerals leads to the plugging of
porous rocks and thus to the downward migration of
the salt seal. As the aseending and laterally migrating
brines generally gain in salt concentration and density,
they finally tend to sink below the basin margins into
the deeper underground (Fig. 6.12a).
Early Cementation
A charaeteristic feature observed in many evaporites is
the oecurrenee of early cementation by gypsum,
anhydrite, halite, and other salt minerals (ef. Fig.
6.11c-e). Primary sedimentary features are best preserved under conditions of early eomplete cementation
(found, e.g., in banded and massive anhydrite beds). If
early cementation is incomplete, nodular and flaser
anhydrite result (Fig. 6.11 b,e). Intensively deformed
anhydrite or gypsum layers exhibiting micro-folds are
usually not caused by sliding, but reflect displacive
growth of the sulfate layer in the subsurface. Some of
these struetures may be related to the transition from
anhydrite to gypsum which needs more volume than its
precursor (tertiary evaporites, see below).
Chapter 6 Special Depositional Environments
Large-Scale Subsurface Dissolution
Stratigraphie correlations and hence also paleogeographie interpretations of aneient salt deposits are
sometimes complieated by substantial subsurfaee dissolution in the zones of active phreatie groundwater
circulation and eompaetion flow.1t may eliminate easily soluble evaporite intervals, preferentially in marginal zones of the basin (Fig. 6.13). Thus, an originally
onlapping sequenee ean be transformed into an apparently offlapping stratigraphie pattern with an erosional
uneonformity. Such a sequenee may be ineorreetly
interpreted as an evaporite system aeeording to the
bull's eye model (Sect. 6.4.2). Salts leaehed from marginal zones of the basin (salted-out halite and potash
salts) are frequently repreeipitated in more eentral
parts of the basin. Highly concentrated NaCI brines
can eause substantial ehanges of the primary mineral
composition of the potash salts.
Tbermobaric Flow Regime and Phase Changes of
Salt Minerals
Already a moderate inerease in temperature (and pressure) induees phase ehanges of several minerals. Hydrated salt minerals and c1ay minerals lose their water
and thus feed a slowly cireulating, initially upward
directed flow system, the so-ealled thermobarie flow
regime (Fig. 6.12a). Primary gypsum is eonverted to
anhydrite at temperatures between 35-45°C if the pore
fluid approaches halite saturation; with lower brine
concentrations this conversion requires higher temperatures (50-60°C) and burial depths of a few hundred
meters. Carnallite releases Mg and water during its
transformation to sylvite (KCI) at 40-50°C. Polyhalite
and kieserite are converted into other minerals at
higher temperatures. In all eases, water is released and
may, if it cannot readily escape, build up excess pore
pressure and promote rock deformation (e.g. small
diapiric structures in sulfate deposits). If volume loss
in the subsurface by subsolution andlor phase changes
occurs concurrently with precipitation of overlying
salts, the basin floor becomes depressed. As a result,
thickened salt deposits are formed locally or in limited
parts ofthe basin (cf. Figs. 6.12.a and 6. 13b).
The impact of the zone of metamorphism on deeply buried
evaporites is not discussed here. The brief remarks to the
processes at shallower burial depth can only indicate and
underline the particularly great importance of diagenetic
processes on the sedimentary structures and secondary mineral composition of evaporites. In order to interpret the origin of ancient evaporites, this substantial diagenetic overprint
has to be taken into account.
Detailed descriptions ofthese phenomena including illustrations and a special nomenclature for the manifold structures, particularly those in calcium sulfate rocks, are given in
the references mr;:ntioned above (e.g., Richter-Bernburg
1985; Langbein 1987; Schreiber 1988a; Warren 1996, 1997).
porosity of the primary sediment is high. Differential
compaction between early cemented marginal and later
cemented central evaporites may somewhat accentuate
differences in bathymetry ofthe depositional basin and
therefore mislead reconstructions of the original basin
topography (cf. Fig. 6.11d). The differential eompaction of early, but only partially eemented calcium sulfate deposits may be one of the processes eausing collapse breccias (Fig. 6.11e). More commonare breecias
resulting from mass flows from topographie highs into
deeper parts of the evaporite basin. Sedimentary breccias also originate from the reworking of salt crusts on
tidal flats.
Sediment compaction expulses pore water out of
both the evaporites and other sediments. As long as the
overlying material is sufficiently permeable, the compaction flow is directed upward. However, salts such
as halite lose their porosity already at shallow burial
depth. Halite crystals generate a kind of mosaic texture
in which the grain boundaries are eurved and display
sutures (dissolution seams). When the overlying salts
act as aseal, the fluids flow laterally updip (Fig.
6.12a).
Along their paths the fluids ean dissolve pre-existing salt minerals, alter older sediments, and preeipitate
new minerals as eement. Dissolution leaves behind
various residues such as evaporite dissolution breccias,
rauhwacken, silicified and calcitized evaporite nodules. Reaction of the fluids with roeks ean cause, for
example, dolomitization of calcarous material or, in
some eases, calcitization of already dolomitized strata.
Cementation by salt minerals leads to the plugging of
porous rocks and thus to the downward migration of
the salt seal. As the aseending and laterally migrating
brines generally gain in salt concentration and density,
they finally tend to sink below the basin margins into
the deeper underground (Fig. 6.12a).
Early Cementation
A charaeteristic feature observed in many evaporites is
the oecurrenee of early cementation by gypsum,
anhydrite, halite, and other salt minerals (ef. Fig.
6.11c-e). Primary sedimentary features are best preserved under conditions of early eomplete cementation
(found, e.g., in banded and massive anhydrite beds). If
early cementation is incomplete, nodular and flaser
anhydrite result (Fig. 6.11 b,e). Intensively deformed
anhydrite or gypsum layers exhibiting micro-folds are
usually not caused by sliding, but reflect displacive
growth of the sulfate layer in the subsurface. Some of
these struetures may be related to the transition from
anhydrite to gypsum which needs more volume than its
precursor (tertiary evaporites, see below).
Chapter 6 Special Depositional Environments
Large-Scale Subsurface Dissolution
Stratigraphie correlations and hence also paleogeographie interpretations of aneient salt deposits are
sometimes complieated by substantial subsurfaee dissolution in the zones of active phreatie groundwater
circulation and eompaetion flow.1t may eliminate easily soluble evaporite intervals, preferentially in marginal zones of the basin (Fig. 6.13). Thus, an originally
onlapping sequenee ean be transformed into an apparently offlapping stratigraphie pattern with an erosional
uneonformity. Such a sequenee may be ineorreetly
interpreted as an evaporite system aeeording to the
bull's eye model (Sect. 6.4.2). Salts leaehed from marginal zones of the basin (salted-out halite and potash
salts) are frequently repreeipitated in more eentral
parts of the basin. Highly concentrated NaCI brines
can eause substantial ehanges of the primary mineral
composition of the potash salts.
Tbermobaric Flow Regime and Phase Changes of
Salt Minerals
Already a moderate inerease in temperature (and pressure) induees phase ehanges of several minerals. Hydrated salt minerals and c1ay minerals lose their water
and thus feed a slowly cireulating, initially upward
directed flow system, the so-ealled thermobarie flow
regime (Fig. 6.12a). Primary gypsum is eonverted to
anhydrite at temperatures between 35-45°C if the pore
fluid approaches halite saturation; with lower brine
concentrations this conversion requires higher temperatures (50-60°C) and burial depths of a few hundred
meters. Carnallite releases Mg and water during its
transformation to sylvite (KCI) at 40-50°C. Polyhalite
and kieserite are converted into other minerals at
higher temperatures. In all eases, water is released and
may, if it cannot readily escape, build up excess pore
pressure and promote rock deformation (e.g. small
diapiric structures in sulfate deposits). If volume loss
in the subsurface by subsolution andlor phase changes
occurs concurrently with precipitation of overlying
salts, the basin floor becomes depressed. As a result,
thickened salt deposits are formed locally or in limited
parts ofthe basin (cf. Figs. 6.12.a and 6. 13b).
The impact of the zone of metamorphism on deeply buried
evaporites is not discussed here. The brief remarks to the
processes at shallower burial depth can only indicate and
underline the particularly great importance of diagenetic
processes on the sedimentary structures and secondary mineral composition of evaporites. In order to interpret the origin of ancient evaporites, this substantial diagenetic overprint
has to be taken into account.
Detailed descriptions ofthese phenomena including illustrations and a special nomenclature for the manifold structures, particularly those in calcium sulfate rocks, are given in
the references mr;:ntioned above (e.g., Richter-Bernburg
1985; Langbein 1987; Schreiber 1988a; Warren 1996, 1997).