272
acteristic (rounded) values and variations in thickness
are frequently observed:
- Carbonates (Stinkkalke) 0.05 mm (0.04-0.1 mm).
- Gypsum and anhydrite 0.5 mm (0.2-30 mm).
- Halite
50 mm (20-150 mm).
The mean ratio of these varve thicknesses is approximately
1: 10:1000 and reflects the solubility ofthese components as
weH as their rate of precipitation when a certain height of
water evaporates. If, for example, the brine has just reached
saturation for CaS04, the evaporation of I m of water can
precipitate about 1 mm anhydrite. This corresponds to the
annual water loss of NaCI-saturated brines in wann arid
regions (Sonnenfeld and Perthuisot 1989). In the case of a
NaCI-saturated brine, the same rate of evaporation generates
a halite layer about 15 cm thick. If the brine is somewhat
undersaturated with respect to these salts, the same annual
evaporation renders thinner salt laminae.
Furthermore, it was pointed out that evaporite varves may
show the 11 year solar cycle (Richter-Bernburg 1960). In the
Zechstein basin, anhydrite varves have been correlated over
distances ofup to 300 km, but in vertical sections, the varved
units are frequently interrupted by diageneticalIy overprinted
units (Fig. 6.11 b) or by erosional surfaces. In many cases it
therefore remains problematic to determine the time span for
the deposition of a thick evaporite sequence with the aid of
varve counts.
The sedimentation rates for the different evaporites
can be detennined in undisturbed, varved evaporite
sections. Using the above data, one obtains the following orders of magnitude:
- Carbonates
- Gypsum and anhydrite
- Halite
5 cm/ka
50 cm/ka
5000 cm/ka
The sedimentation rate for evaporitic carbonates is in
the same order as nonnal marine shelf carbonates (cf.
Sect. 3.4). However; the deposition of marine salts,
especially that of halite and the potash salts, can be
extremely fast in comparison to other marine sediments. Sabkha evaporites, mainly consisting of
anhydrite and carbonate, show vertical growth rates of
the order of I m/ka and horizontal pro gradation of I
km/ka (Schreiber and Hsü 1980).
Of course, alI these values vary considerably within a certain
evaporite sequence or from one salt deposit to another. The
high rates for the evaporitic salts are only valid for basins
where the whole water column has reached saturation for the
Fig. 6.11. a Thickness of annual varves in halite,
anhydrite, and carbonate. b Anhydrite section of
Zechstein Basin (cf. Fig. 6.10; cycle Zl, Goslar)
showing repeated facies change due to variations in
brine concentration and diagenesis, see c. (The terms
for the description of sedimentary structures are not
generally used). c Vertical facies change in sulfates
with increasing brine concentration and enhanced
ear1y cementation. d Early cementation of marginal
Chapter 6 Special Depositional Environments
corresponding salt to be precipitated. Prior to this situation
or as a result of episodic brine dilution, the sedimentation
rates can be considerably lower.
Nevertheless, the assumption that a rather deep basin can
be rapidly filled up with a few halite cycles appears to be
welI justified by t!1e observations on varves. If, for example,
a 1000 m deep basin, already saturated with respect to halite,
is cut off from sea water influx, desiccation ofthis basin can
produce a halite layer of about 150 m in thickness throughout the entire basin in a time period of some thousands of
years. If halite precipitation occurs only in parts of the
shrinking basin, the halite deposit can become much thicker.
Such a rapid deposition of halite is possible only in a preexisting deep basin, because subsidence cannot proceed with
the same high rate.
The rapid evaporite deposition from concentrated
brines supports the deep basin model discussed above
for "saline giants". However, the deposition of thick
units of gypsum and anhydrite requires an open brine
reflux system, because the solubility of CaS0 4 is too
low to allow an adequate storage of this component in
the brine of a closed basin. In the case of cyclic
evaporite sequences, most of the geologie time comprises the intervals of non-evaporite deposition.
6.4.5 Sequences and Sedimentary Structures
of Evaporites
Progressing or decreasing brine concentrations cause
characteristic vcrtical successions of salt deposits including their accompanying sediments such as marine
bituminous shales, carbonates, and terrestrial beds.
Some examples are demonstrated in Figs. 6.4e,f, 6.5c,
6.7, and 6.8c-e. These simplified salt successions neglect the possibility of microbial sulfate reduction
which can remove at least limited quantities of gypsum
settling together with organic matter.
This process can playa role in an early, more or less euxinic
stage of a salt basin development (Sect. 6.4.2) when the
precipitation of carbonate is replaced by gypsum. Then the
beginning ofthe sulfate precipitation may be obliterated or,
as observed in some cases, halite directly overlies bituminous
marls or carbonates.
Correlation of vertical salt sequences is possible to
some extent only in the central parts of relatively deep
basins. Coeval sediments along the basin margins nor
gypsum (later converted to anhydrite) causes differential compaction and some change in basin topography. e Collapse breccia caused by early cementation
of top layers and subsequent differential compaction.
f Cross section of Zechstein Basin showing lateral
facies change during two epaporite cycles. Facies
boundaries do not necessarily indicate time lines (cf.
Fig. 7.25). (a,b,f based on Richter-Bemburg 1985;
c-e after Langbein 1987)
acteristic (rounded) values and variations in thickness
are frequently observed:
- Carbonates (Stinkkalke) 0.05 mm (0.04-0.1 mm).
- Gypsum and anhydrite 0.5 mm (0.2-30 mm).
- Halite
50 mm (20-150 mm).
The mean ratio of these varve thicknesses is approximately
1: 10:1000 and reflects the solubility ofthese components as
weH as their rate of precipitation when a certain height of
water evaporates. If, for example, the brine has just reached
saturation for CaS04, the evaporation of I m of water can
precipitate about 1 mm anhydrite. This corresponds to the
annual water loss of NaCI-saturated brines in wann arid
regions (Sonnenfeld and Perthuisot 1989). In the case of a
NaCI-saturated brine, the same rate of evaporation generates
a halite layer about 15 cm thick. If the brine is somewhat
undersaturated with respect to these salts, the same annual
evaporation renders thinner salt laminae.
Furthermore, it was pointed out that evaporite varves may
show the 11 year solar cycle (Richter-Bernburg 1960). In the
Zechstein basin, anhydrite varves have been correlated over
distances ofup to 300 km, but in vertical sections, the varved
units are frequently interrupted by diageneticalIy overprinted
units (Fig. 6.11 b) or by erosional surfaces. In many cases it
therefore remains problematic to determine the time span for
the deposition of a thick evaporite sequence with the aid of
varve counts.
The sedimentation rates for the different evaporites
can be detennined in undisturbed, varved evaporite
sections. Using the above data, one obtains the following orders of magnitude:
- Carbonates
- Gypsum and anhydrite
- Halite
5 cm/ka
50 cm/ka
5000 cm/ka
The sedimentation rate for evaporitic carbonates is in
the same order as nonnal marine shelf carbonates (cf.
Sect. 3.4). However; the deposition of marine salts,
especially that of halite and the potash salts, can be
extremely fast in comparison to other marine sediments. Sabkha evaporites, mainly consisting of
anhydrite and carbonate, show vertical growth rates of
the order of I m/ka and horizontal pro gradation of I
km/ka (Schreiber and Hsü 1980).
Of course, alI these values vary considerably within a certain
evaporite sequence or from one salt deposit to another. The
high rates for the evaporitic salts are only valid for basins
where the whole water column has reached saturation for the
Fig. 6.11. a Thickness of annual varves in halite,
anhydrite, and carbonate. b Anhydrite section of
Zechstein Basin (cf. Fig. 6.10; cycle Zl, Goslar)
showing repeated facies change due to variations in
brine concentration and diagenesis, see c. (The terms
for the description of sedimentary structures are not
generally used). c Vertical facies change in sulfates
with increasing brine concentration and enhanced
ear1y cementation. d Early cementation of marginal
Chapter 6 Special Depositional Environments
corresponding salt to be precipitated. Prior to this situation
or as a result of episodic brine dilution, the sedimentation
rates can be considerably lower.
Nevertheless, the assumption that a rather deep basin can
be rapidly filled up with a few halite cycles appears to be
welI justified by t!1e observations on varves. If, for example,
a 1000 m deep basin, already saturated with respect to halite,
is cut off from sea water influx, desiccation ofthis basin can
produce a halite layer of about 150 m in thickness throughout the entire basin in a time period of some thousands of
years. If halite precipitation occurs only in parts of the
shrinking basin, the halite deposit can become much thicker.
Such a rapid deposition of halite is possible only in a preexisting deep basin, because subsidence cannot proceed with
the same high rate.
The rapid evaporite deposition from concentrated
brines supports the deep basin model discussed above
for "saline giants". However, the deposition of thick
units of gypsum and anhydrite requires an open brine
reflux system, because the solubility of CaS0 4 is too
low to allow an adequate storage of this component in
the brine of a closed basin. In the case of cyclic
evaporite sequences, most of the geologie time comprises the intervals of non-evaporite deposition.
6.4.5 Sequences and Sedimentary Structures
of Evaporites
Progressing or decreasing brine concentrations cause
characteristic vcrtical successions of salt deposits including their accompanying sediments such as marine
bituminous shales, carbonates, and terrestrial beds.
Some examples are demonstrated in Figs. 6.4e,f, 6.5c,
6.7, and 6.8c-e. These simplified salt successions neglect the possibility of microbial sulfate reduction
which can remove at least limited quantities of gypsum
settling together with organic matter.
This process can playa role in an early, more or less euxinic
stage of a salt basin development (Sect. 6.4.2) when the
precipitation of carbonate is replaced by gypsum. Then the
beginning ofthe sulfate precipitation may be obliterated or,
as observed in some cases, halite directly overlies bituminous
marls or carbonates.
Correlation of vertical salt sequences is possible to
some extent only in the central parts of relatively deep
basins. Coeval sediments along the basin margins nor
gypsum (later converted to anhydrite) causes differential compaction and some change in basin topography. e Collapse breccia caused by early cementation
of top layers and subsequent differential compaction.
f Cross section of Zechstein Basin showing lateral
facies change during two epaporite cycles. Facies
boundaries do not necessarily indicate time lines (cf.
Fig. 7.25). (a,b,f based on Richter-Bemburg 1985;
c-e after Langbein 1987)