2.5 Lake Sediments
floods. Then part of the salt is redissolved and later
reprecipitated in the salt pan on top of silty and
c1ayey layers resulting from the flood event. Reworked carbonate crusts and microbial mats may 10cally form intrac1ast breccias or flat pebble conglomerates.
The salts dissolved during floods and concentrated
in depressions having no outflow can also be leached
by infiltrating rain water and enhance the rnineralization of groundwater.
This is the case when the groundwater level as weil as its
capillary fringe drop below the lake floor, as for example
reported from a number of Quatemary lakes in southeastem
Australia (Bowler 1986; Bowler et al. 1986; Teller and
Last 1990; Rosen 1994). Then vegetation can begin to
grow and form some soi! or duricrust (e.g. calcrete) on the
lake floor. Figure 2.34 shows the interaction between a
fluctuating groundwater level and the sedimentary proces ses in such a lake. It can be seen that the lake sediments
respond very sensitively to slight changes in the hydrologie
regime of the lake and thus also to minor variations in cl imate. They frequently exhibit minor sedimentary cycles of
widely differing thickness and duration. If leaching of salts
and deflation are involved, such cycles tend to become
asymmetrie (Fig. 2.34).
In such cases, a soil horizon developed on the dry
lake floor corresponds with the lowermost groundwater table and therefore most likely with the dryest
period (Fig. 2.34) rather than an underlying or overlying salt layer. For that reason, salt layers or soil
horizons of distant lakes should be correlated with
caution, if their hydrological regimes are not equal.
It seems that extensive fossil red c1aystones and
marlstones, deposited in large topographie depressions with one or several playa lakes, are not very
well studied (e.g. Rosen 1994; Gaupp 1997). During
the Earth's history such systems have accumulated
great volumes of fine-grained continental sediments
(e.g. in the Permian and Triassie). However, modem
analogs of such systems are mostly limited in size.
A widely extended modem example is the drainage basin
of Lake Eyre in central Australia (Croke et al. 1998) which
covers an area in the order of 1 x 10 6 km 2 • In the Tertiary
and Quatemary, fluvial and deltaic-Iacustrine facies alternated several times in the central part of the basin as a result of climate change and variation in sediment supply.
Arid phases caused in places deflation.
Especially large depositional environments of this
type respond sensitively to c1irnate change, and many
sediments of ancient examples show a distinct
cyc1icity (cf. Sect. 7.7). Episodic flooding of lowrelief playa plains, phases of drying, dessication and
pedogenesis generate strata which can be traced over
long distances. Apart from fluvial transport, eolian
dust can significantly contribute to the sediment
buildup.
87
Drying lakes with silty-c1ayey sediments, inc1uding some salts, also favor the formation of "c1ay pellets" on the lake floor which are blown away by wind
to form clay dunes (or lunettes, cf. Sect. 2.3.4) along
the rim of the lake (cf. Fig. 2.28b4). If a lake is exposed to this deflation process for some time, its
floor is lowered and its sedimentary record interrupted.
Examples of Closed Lake Basins
and Their Sediments
Apart fr0m the examples mentioned briefly in the
previous text and further below, the most important
features of some c10sed lake basins are discussed
here separately. The examples follow in the order of
increasing salinity of the basins, but other aspects are
also considered.
Caspian Sea. This inland sea is the world's largest
modem closed lake basin. Its water is brackish (13
g/l dissolved species) and its sediments are dominated by detrital clastics. These are delivered by the
river V olga from the north and several middle-sized
rivers from the west and south (cf. Fig. 11. 7b). In
addition, production of autochthonous bio genie and
bio-induced carbonate is significant, particularly in
the littoral zone and in the shallow northem part of
the basin. Here, sediments rich in carbonate, including shellbeds and oolites, are comrnon. In the 800 to
1000 m deep central and southem part of the lake,
fine-grained marls accumulate with a very high sedimentation rate on the order of 1 m/ka (locally up to
10 mlka; cf. Sect. 11.2.3).
Because these sediments are buried rapidly, their high content in organic matter is both weil preserved and
diagenetically mature in terms of hydrocarbon generation.
They therefore enable oil and gas production from Pliocene
to Quatemary strata in the Baku area. Irregularly occurring
gas eruptions and numerous mud volcanoes testify to the
fact that oil and gas generating processes, as weil as differential compaction of these young sediments, are still in
operation (see also Sect. 14.2).
In the context of this chapter, the large, shallow lagoon, the Kara Bogas Goi, on the eastem margin of
the Caspian Sea is of particular interest (e.g.,
Sonnenfeld 1984; Müller 1988). As a result of very
high evaporation in the warm, arid climate, the water
level of the lagoon drops below that of the Caspian
Sea and thus causes continuous inflow of brackish
water. The ion concentration of this water increases
from the inlet to the inner part of the lagoon and
hence leads to the precipitation of carbonate, gypsum, glauberite and halite in a lateral succession (cf.
Sect. 6.4.2).
floods. Then part of the salt is redissolved and later
reprecipitated in the salt pan on top of silty and
c1ayey layers resulting from the flood event. Reworked carbonate crusts and microbial mats may 10cally form intrac1ast breccias or flat pebble conglomerates.
The salts dissolved during floods and concentrated
in depressions having no outflow can also be leached
by infiltrating rain water and enhance the rnineralization of groundwater.
This is the case when the groundwater level as weil as its
capillary fringe drop below the lake floor, as for example
reported from a number of Quatemary lakes in southeastem
Australia (Bowler 1986; Bowler et al. 1986; Teller and
Last 1990; Rosen 1994). Then vegetation can begin to
grow and form some soi! or duricrust (e.g. calcrete) on the
lake floor. Figure 2.34 shows the interaction between a
fluctuating groundwater level and the sedimentary proces ses in such a lake. It can be seen that the lake sediments
respond very sensitively to slight changes in the hydrologie
regime of the lake and thus also to minor variations in cl imate. They frequently exhibit minor sedimentary cycles of
widely differing thickness and duration. If leaching of salts
and deflation are involved, such cycles tend to become
asymmetrie (Fig. 2.34).
In such cases, a soil horizon developed on the dry
lake floor corresponds with the lowermost groundwater table and therefore most likely with the dryest
period (Fig. 2.34) rather than an underlying or overlying salt layer. For that reason, salt layers or soil
horizons of distant lakes should be correlated with
caution, if their hydrological regimes are not equal.
It seems that extensive fossil red c1aystones and
marlstones, deposited in large topographie depressions with one or several playa lakes, are not very
well studied (e.g. Rosen 1994; Gaupp 1997). During
the Earth's history such systems have accumulated
great volumes of fine-grained continental sediments
(e.g. in the Permian and Triassie). However, modem
analogs of such systems are mostly limited in size.
A widely extended modem example is the drainage basin
of Lake Eyre in central Australia (Croke et al. 1998) which
covers an area in the order of 1 x 10 6 km 2 • In the Tertiary
and Quatemary, fluvial and deltaic-Iacustrine facies alternated several times in the central part of the basin as a result of climate change and variation in sediment supply.
Arid phases caused in places deflation.
Especially large depositional environments of this
type respond sensitively to c1irnate change, and many
sediments of ancient examples show a distinct
cyc1icity (cf. Sect. 7.7). Episodic flooding of lowrelief playa plains, phases of drying, dessication and
pedogenesis generate strata which can be traced over
long distances. Apart from fluvial transport, eolian
dust can significantly contribute to the sediment
buildup.
87
Drying lakes with silty-c1ayey sediments, inc1uding some salts, also favor the formation of "c1ay pellets" on the lake floor which are blown away by wind
to form clay dunes (or lunettes, cf. Sect. 2.3.4) along
the rim of the lake (cf. Fig. 2.28b4). If a lake is exposed to this deflation process for some time, its
floor is lowered and its sedimentary record interrupted.
Examples of Closed Lake Basins
and Their Sediments
Apart fr0m the examples mentioned briefly in the
previous text and further below, the most important
features of some c10sed lake basins are discussed
here separately. The examples follow in the order of
increasing salinity of the basins, but other aspects are
also considered.
Caspian Sea. This inland sea is the world's largest
modem closed lake basin. Its water is brackish (13
g/l dissolved species) and its sediments are dominated by detrital clastics. These are delivered by the
river V olga from the north and several middle-sized
rivers from the west and south (cf. Fig. 11. 7b). In
addition, production of autochthonous bio genie and
bio-induced carbonate is significant, particularly in
the littoral zone and in the shallow northem part of
the basin. Here, sediments rich in carbonate, including shellbeds and oolites, are comrnon. In the 800 to
1000 m deep central and southem part of the lake,
fine-grained marls accumulate with a very high sedimentation rate on the order of 1 m/ka (locally up to
10 mlka; cf. Sect. 11.2.3).
Because these sediments are buried rapidly, their high content in organic matter is both weil preserved and
diagenetically mature in terms of hydrocarbon generation.
They therefore enable oil and gas production from Pliocene
to Quatemary strata in the Baku area. Irregularly occurring
gas eruptions and numerous mud volcanoes testify to the
fact that oil and gas generating processes, as weil as differential compaction of these young sediments, are still in
operation (see also Sect. 14.2).
In the context of this chapter, the large, shallow lagoon, the Kara Bogas Goi, on the eastem margin of
the Caspian Sea is of particular interest (e.g.,
Sonnenfeld 1984; Müller 1988). As a result of very
high evaporation in the warm, arid climate, the water
level of the lagoon drops below that of the Caspian
Sea and thus causes continuous inflow of brackish
water. The ion concentration of this water increases
from the inlet to the inner part of the lagoon and
hence leads to the precipitation of carbonate, gypsum, glauberite and halite in a lateral succession (cf.
Sect. 6.4.2).