84
1972). In many modern to P1eistocene 1akes, dolomite may have formed as primary mineral (Last
1990). Ostracods, algae and bacteria, all of them to1erating marked changes in salinity, contribute to the
formation of carbonate deposits (e.g. ostracod limestones, algal bioherms, stromatolites, etc.).
In perennial lakes with inflow balancing evaporation, the ion concentration of the water increases
with time. Then the resulting sedimentary sequences
in the lake center show an evolution from calcareous
to dolomitic marls, which may be rich in organic matter, and finally to evaporites (Fig. 2.32e). If such a
long-term trend is superimposed by shorter term climatic variations, the resulting sequences frequently
display rhythmic or cyclic phenomena, for example
marls alternating with carbonate layers, or kerogenrich marls alternating with claystones or marls less
rich in organic matter (Fig. 2.32t).
An alternative to this model is a slowly dropping
lake level 1eading likewise to an increase in ion concentration of the shrinking water body. Consequently, the highest elevated littoral zone is characterized by biogenic and bio-induced carbonate deposited under non-saline conditions (Fig. 2.32b). This
deposit may alternate with fluvial sands, beach sands,
or alluvial fans. In stages 2 and 3 (Fig. 2.32c and d)
of the lake evolution, the littoral zone migrates
basinward and rests on sediments deposited during a
higher lake level. Thus, shallowing-upward sequences are generated which also indicate an upward
increase in salinity. In a cross-section from the marginal, littoral zone to the center of the lake (Fig.
2.32b to d), the carbonate facies changes from that of
a nonsaline to a highly saline lake. Marginal, calcareous ooids and desiccation cracks and tepees in chemically precipitated micritic carbonate mud are common (Sect. 3.2.2).
Siderite concretions, often representing a mixture of several carbonate minerals (containing Fe, Mg, Ca, Mn) is
fairly common in ancient lake and brackish water deposits
but rare in the sediments of modem lakes. Siderite can
form diagenetically under reducing conditions where the
calcium and sulfide contents of pore waters are low and, in
addition, the Fe/Ca mol ratio is relatively high (,,0.4;
Füchtbauer and Richter 1988). Since the water of many
lakes contains much less sulfate than sea watet, the source
for generating iron sulfide with the aid of sulfate reducing
bacteria is limited. Therefore, iron carbonate has a better
chance of being formed in lake sediments than in marine
sediments, provided iron-bearing minerals can release sufficient iron.
Sulfate Lakes, Soda Lakes, Chloride Lakes
After precipitation of gypsum and possibly other sulfate minerals, the remaining lake water contains only
salts of very high solubility. In order to precipitate
these components, i.e., predominantly Na, Mg, Cl,
Chapter 2 Continental Sediments
and S04, a further drastic increase in the total ion
concentration of the lake water is required (for halite
precipitation 390 to 440 g/kg of solution). The nature
and succession of salts deposited after this process is
controlled by the chemistry of the lake water (cf. Fig.
2.31). The most prominent salt occurring in most of
the lakes at this stage is halite (NaCl) (Fig. 2.33a). It
may be accompanied by Mg salts and some other
compounds which are rarely preserved in the ancient
record.
Saline waters rich in sodium and sulfate precipitate thenardite (N~S04)' mirabilite (N~S041 OH20),
and glauberite (CaS04N~S04) before halite can
form. Soda lakes precipitate natron (N a 2 C0 3 1 OHP)
and trona (NaHC03N~C032H20) (Fig. 2.33a2 and
a3). As a result of increasing aridity these lakes may
develop the following sequence (from top to bottom):
Trona and gaylussite (Na 2 C0 3 CaC0 3 5H 2 0,
formed diagenetically).
Dolomite and gaylussite (in summer).
- Calcite and dolomite.
Prior to the precipitation of trona, the ion concentrati on of the brine must be very high. If this is reached,
the comparatively low evaporation loss of one year
can cause precipitation of a fairly thick trona layer on
the order of 5 mm. Similarly, annual layers of halite
(rock salt) attain a thickness on the order of 1 cm (cf.
Sect. 6.4).
Playa Lakes (Inland Sabkhas)
Whereas perennial, long-persisting salt lakes can accumulate rather thick and pure salt deposits (cf. Fig.
2.28b2), ephemeral playa lakes usually leave behind
only thin salt layers alternating with clastic material.
These playa lakes are dry most of the time, with a salt
pan in their center (Figs. 2.28b3 and 2.33b). Their
marginal zone is commonly characterized by wide
sand and mud flats, which may develop thin salt
crnsts (consisting, e.g., of proto-dolomite on the
outer flats and gypsum and halite on the inner flats).
In addition, this zone shows desiccation cracks and
locally also some sparse vegetation and burrowing
organisms (Fig. 2.33b 1). Weil laminated microbial
mats are fairly common on the mud flats, but some
centimeters or decimeters below the surface they are
mostly destroyed.
As long as the capillary fringe of the groundwater
table reaches the lake floor, salts such as halite, gypsum, and sodium sulfate crystallize interstitially from
the ascending groundwater. This process is referred
to as evaporative pumping. It leads to the formation
of nodules or irregular layers of these salts below the
surface of sand and mud flats. The flats and the central salt pan are occasionally inundated by sheet
1972). In many modern to P1eistocene 1akes, dolomite may have formed as primary mineral (Last
1990). Ostracods, algae and bacteria, all of them to1erating marked changes in salinity, contribute to the
formation of carbonate deposits (e.g. ostracod limestones, algal bioherms, stromatolites, etc.).
In perennial lakes with inflow balancing evaporation, the ion concentration of the water increases
with time. Then the resulting sedimentary sequences
in the lake center show an evolution from calcareous
to dolomitic marls, which may be rich in organic matter, and finally to evaporites (Fig. 2.32e). If such a
long-term trend is superimposed by shorter term climatic variations, the resulting sequences frequently
display rhythmic or cyclic phenomena, for example
marls alternating with carbonate layers, or kerogenrich marls alternating with claystones or marls less
rich in organic matter (Fig. 2.32t).
An alternative to this model is a slowly dropping
lake level 1eading likewise to an increase in ion concentration of the shrinking water body. Consequently, the highest elevated littoral zone is characterized by biogenic and bio-induced carbonate deposited under non-saline conditions (Fig. 2.32b). This
deposit may alternate with fluvial sands, beach sands,
or alluvial fans. In stages 2 and 3 (Fig. 2.32c and d)
of the lake evolution, the littoral zone migrates
basinward and rests on sediments deposited during a
higher lake level. Thus, shallowing-upward sequences are generated which also indicate an upward
increase in salinity. In a cross-section from the marginal, littoral zone to the center of the lake (Fig.
2.32b to d), the carbonate facies changes from that of
a nonsaline to a highly saline lake. Marginal, calcareous ooids and desiccation cracks and tepees in chemically precipitated micritic carbonate mud are common (Sect. 3.2.2).
Siderite concretions, often representing a mixture of several carbonate minerals (containing Fe, Mg, Ca, Mn) is
fairly common in ancient lake and brackish water deposits
but rare in the sediments of modem lakes. Siderite can
form diagenetically under reducing conditions where the
calcium and sulfide contents of pore waters are low and, in
addition, the Fe/Ca mol ratio is relatively high (,,0.4;
Füchtbauer and Richter 1988). Since the water of many
lakes contains much less sulfate than sea watet, the source
for generating iron sulfide with the aid of sulfate reducing
bacteria is limited. Therefore, iron carbonate has a better
chance of being formed in lake sediments than in marine
sediments, provided iron-bearing minerals can release sufficient iron.
Sulfate Lakes, Soda Lakes, Chloride Lakes
After precipitation of gypsum and possibly other sulfate minerals, the remaining lake water contains only
salts of very high solubility. In order to precipitate
these components, i.e., predominantly Na, Mg, Cl,
Chapter 2 Continental Sediments
and S04, a further drastic increase in the total ion
concentration of the lake water is required (for halite
precipitation 390 to 440 g/kg of solution). The nature
and succession of salts deposited after this process is
controlled by the chemistry of the lake water (cf. Fig.
2.31). The most prominent salt occurring in most of
the lakes at this stage is halite (NaCl) (Fig. 2.33a). It
may be accompanied by Mg salts and some other
compounds which are rarely preserved in the ancient
record.
Saline waters rich in sodium and sulfate precipitate thenardite (N~S04)' mirabilite (N~S041 OH20),
and glauberite (CaS04N~S04) before halite can
form. Soda lakes precipitate natron (N a 2 C0 3 1 OHP)
and trona (NaHC03N~C032H20) (Fig. 2.33a2 and
a3). As a result of increasing aridity these lakes may
develop the following sequence (from top to bottom):
Trona and gaylussite (Na 2 C0 3 CaC0 3 5H 2 0,
formed diagenetically).
Dolomite and gaylussite (in summer).
- Calcite and dolomite.
Prior to the precipitation of trona, the ion concentrati on of the brine must be very high. If this is reached,
the comparatively low evaporation loss of one year
can cause precipitation of a fairly thick trona layer on
the order of 5 mm. Similarly, annual layers of halite
(rock salt) attain a thickness on the order of 1 cm (cf.
Sect. 6.4).
Playa Lakes (Inland Sabkhas)
Whereas perennial, long-persisting salt lakes can accumulate rather thick and pure salt deposits (cf. Fig.
2.28b2), ephemeral playa lakes usually leave behind
only thin salt layers alternating with clastic material.
These playa lakes are dry most of the time, with a salt
pan in their center (Figs. 2.28b3 and 2.33b). Their
marginal zone is commonly characterized by wide
sand and mud flats, which may develop thin salt
crnsts (consisting, e.g., of proto-dolomite on the
outer flats and gypsum and halite on the inner flats).
In addition, this zone shows desiccation cracks and
locally also some sparse vegetation and burrowing
organisms (Fig. 2.33b 1). Weil laminated microbial
mats are fairly common on the mud flats, but some
centimeters or decimeters below the surface they are
mostly destroyed.
As long as the capillary fringe of the groundwater
table reaches the lake floor, salts such as halite, gypsum, and sodium sulfate crystallize interstitially from
the ascending groundwater. This process is referred
to as evaporative pumping. It leads to the formation
of nodules or irregular layers of these salts below the
surface of sand and mud flats. The flats and the central salt pan are occasionally inundated by sheet