2.5 Lake Sediments
Such successions have been found, e.g., in the subsurface
of modem playas of the Great Basin in North America.
Drilling in Owens Lake, Searles Lake and Death Valley,
California, has revealed repeated depositional sequences
ranging from the sediments of mudflats via those of a saline pan with ephemeral lakes to sediments of a perennial
lake with fluctuating water level and varying salinity (Bischoff et al. 1997; Lowenstein et al. 1999). This successions
reflect the change from dry and warm to wetter and colder
climate which occurred in time periods of roughly 100 ka
in the late Quaternary. In Death Valley one of these sedimentary cycles is about 100 m in thickness. Specific studies on ostracodes and diatoms as weil as stable isotopes in
lake carbonates support this interpretation. Fluid inclusions
in halite crystals were used to determine the original water
temperature (homogenization temperature) during salt crystallization; sediments rich in plagioclase rock flower (in
cores drilled in Owens lake) indicate glacial advance in the
Sierra Nevada during periods ofwet climate.
A similar example ofPleistocene lake-level fluctuations
and cyclic evaporite deposits was reported from one of the
Salars of northwest Argentina (Vandervoort 1997).
Frequently cited Mesozoic examples are those of the
classic Newark supergroup (Triassic-Jurassic) in eastern
North America (van Houten 1964; Gore 1989; see also
Sects. 7.6 and 11.3.3; Fig. 12.7b) and the marl-carbonategypsum-halite successions ofthe Triassie (Keuper) basin in
Central and Western Europe (e.g., Schröder 1982; Aigner
and Bachmann 1992; cf. Sect. 12.3.2 and Fig. 12.17), parts
ofwhich were repeatedly filled by prograding birdfoot deltas or wider alluvial plains (Wurster 1964).
Relative lake level changes and sequences in open lakes
of humid regions seem to be mainly controlled by
tectonism and changes in sediment supply rather than by
change of the hydrologie budget. An example of this category is Lake Baikai in Siberia (Scholz et al. 1998).
For a number of large, long-persisting individual lakes,
composite facies models are required which combine aspects of both the open and the closed lake systems (e.g.,
Sullivan 1985; Gore 1989). Arid lake basins close to the
sea may receive influx from both fresh-water rivers and the
ocean, resulting in a complicated system which precipitates
various carbonates and evaporites (e.g., Decima et al.
1988).
2.5.7 Specific Features of Lake Sediments
Sedimentation Rates and Lifetime of Lakes
Most of the present-day peri-Alpine open lake systems are fed by rivers draining mountainous regions.
They therefore show very high sedimentation rates in
the order of 1 to 10 mika and more (cf. Sect. 11.2.3).
Such lakes, particularly small and shallow ones, have
only a comparatively short lifetime in the order of 10
ka to 100 ka. This also applies to many lakes located
in graben structures and rift zones bordered by
mountain ranges. Even shorter lifetimes have icecontact lakes and proglacial lakes (Sect. 2.1) which
may become filled within a few thousand years or
less and therefore rarely experience long-term drastic
climatic changes.
91
Large and deep lake basins with subsiding basin
floor have, of course, long lifetimes. This is true of
large rift basins such as Lake Baikai and several
lakes of the East African Rift as well as of some
lakes in continental sag basins (e.g. Lake Eyre in
Australia) and other lakes of tectonic origin (e.g. L.
Maracaibo in Venezuela). All these lakes exist since
at least 20 Ma (Meybeck 1995).
In the southern Caspian Sea, representing a backare basin,
about 1 mika is deposited, which signifies that Quaternary
lake sediments reach a thickness of more than 1 km. Such
high sedimentation rates are capable of compensating even
for rapid subsidence (cf. Chap. 8).
Sedimentation rates of less than 0.1 mika are typical
of large lakes in lowland regions when their ratio of
drainage area to lake area is low. Then they receive
only limited quantities ofpredominantly fine-grained,
clastic material which is widely distributed over the
lake area. Such lakes can persist for long time periods, provided their floor subsides sufficiently. Then,
carbonates may constitute a significant sediment
component.
Lacustrine salts can be precipitated rapidly (several meters to tens of meters per 1000 years), provided the ion concentration of perenniallakes or that
of groundwater below an ephemeral lake has already
reached saturation with respect to the principal mineral phases. Then, further small evaporation losses
cause rapid salt precipitation. U sing the data for
chemical (and mechanical) denudation rates (cf. Sect.
9.2), one can estimate the time necessary for the accumulation of salts (and detrital clastics) in a lake
basin representing a certain fraction of the total
drainage area (cf. Sects. 11.2 and 11.4).
Isotopes and Fluid Inclusions
Indicating Paleoclimate
The paleoenvironmental interpretation of ancient
lake basins can be refined by stable isotope studies
(e.g. Oberhänsli and Allen 1987; Talbot and Kelts
1989; Marshall 1992; Gat 1995). The heavy 18 0 in
authogenic carbonate and shells of organisms records
the isotopic composition of the paleolake water and,
in closed systems, its state of evaporation. In open
systems, the isotopic signature of lake carbonate reflects to a large degree the properties of river inflow
and thus the climate and altitude of the drainage area.
The primary isotope signal can be obscured by the
effects of diagenetic alteration, especially that of the
oxygen isotopes. For all these reasons, oxygen isotope values are sometimes difficult to interpret.
Relative enrichment of the heavy carbon isotope
13C indicates that plant material was strongly affected
by bacterial activity and that the lake floor possibly
emerged. Isotope studies also reveal that primary lake
Such successions have been found, e.g., in the subsurface
of modem playas of the Great Basin in North America.
Drilling in Owens Lake, Searles Lake and Death Valley,
California, has revealed repeated depositional sequences
ranging from the sediments of mudflats via those of a saline pan with ephemeral lakes to sediments of a perennial
lake with fluctuating water level and varying salinity (Bischoff et al. 1997; Lowenstein et al. 1999). This successions
reflect the change from dry and warm to wetter and colder
climate which occurred in time periods of roughly 100 ka
in the late Quaternary. In Death Valley one of these sedimentary cycles is about 100 m in thickness. Specific studies on ostracodes and diatoms as weil as stable isotopes in
lake carbonates support this interpretation. Fluid inclusions
in halite crystals were used to determine the original water
temperature (homogenization temperature) during salt crystallization; sediments rich in plagioclase rock flower (in
cores drilled in Owens lake) indicate glacial advance in the
Sierra Nevada during periods ofwet climate.
A similar example ofPleistocene lake-level fluctuations
and cyclic evaporite deposits was reported from one of the
Salars of northwest Argentina (Vandervoort 1997).
Frequently cited Mesozoic examples are those of the
classic Newark supergroup (Triassic-Jurassic) in eastern
North America (van Houten 1964; Gore 1989; see also
Sects. 7.6 and 11.3.3; Fig. 12.7b) and the marl-carbonategypsum-halite successions ofthe Triassie (Keuper) basin in
Central and Western Europe (e.g., Schröder 1982; Aigner
and Bachmann 1992; cf. Sect. 12.3.2 and Fig. 12.17), parts
ofwhich were repeatedly filled by prograding birdfoot deltas or wider alluvial plains (Wurster 1964).
Relative lake level changes and sequences in open lakes
of humid regions seem to be mainly controlled by
tectonism and changes in sediment supply rather than by
change of the hydrologie budget. An example of this category is Lake Baikai in Siberia (Scholz et al. 1998).
For a number of large, long-persisting individual lakes,
composite facies models are required which combine aspects of both the open and the closed lake systems (e.g.,
Sullivan 1985; Gore 1989). Arid lake basins close to the
sea may receive influx from both fresh-water rivers and the
ocean, resulting in a complicated system which precipitates
various carbonates and evaporites (e.g., Decima et al.
1988).
2.5.7 Specific Features of Lake Sediments
Sedimentation Rates and Lifetime of Lakes
Most of the present-day peri-Alpine open lake systems are fed by rivers draining mountainous regions.
They therefore show very high sedimentation rates in
the order of 1 to 10 mika and more (cf. Sect. 11.2.3).
Such lakes, particularly small and shallow ones, have
only a comparatively short lifetime in the order of 10
ka to 100 ka. This also applies to many lakes located
in graben structures and rift zones bordered by
mountain ranges. Even shorter lifetimes have icecontact lakes and proglacial lakes (Sect. 2.1) which
may become filled within a few thousand years or
less and therefore rarely experience long-term drastic
climatic changes.
91
Large and deep lake basins with subsiding basin
floor have, of course, long lifetimes. This is true of
large rift basins such as Lake Baikai and several
lakes of the East African Rift as well as of some
lakes in continental sag basins (e.g. Lake Eyre in
Australia) and other lakes of tectonic origin (e.g. L.
Maracaibo in Venezuela). All these lakes exist since
at least 20 Ma (Meybeck 1995).
In the southern Caspian Sea, representing a backare basin,
about 1 mika is deposited, which signifies that Quaternary
lake sediments reach a thickness of more than 1 km. Such
high sedimentation rates are capable of compensating even
for rapid subsidence (cf. Chap. 8).
Sedimentation rates of less than 0.1 mika are typical
of large lakes in lowland regions when their ratio of
drainage area to lake area is low. Then they receive
only limited quantities ofpredominantly fine-grained,
clastic material which is widely distributed over the
lake area. Such lakes can persist for long time periods, provided their floor subsides sufficiently. Then,
carbonates may constitute a significant sediment
component.
Lacustrine salts can be precipitated rapidly (several meters to tens of meters per 1000 years), provided the ion concentration of perenniallakes or that
of groundwater below an ephemeral lake has already
reached saturation with respect to the principal mineral phases. Then, further small evaporation losses
cause rapid salt precipitation. U sing the data for
chemical (and mechanical) denudation rates (cf. Sect.
9.2), one can estimate the time necessary for the accumulation of salts (and detrital clastics) in a lake
basin representing a certain fraction of the total
drainage area (cf. Sects. 11.2 and 11.4).
Isotopes and Fluid Inclusions
Indicating Paleoclimate
The paleoenvironmental interpretation of ancient
lake basins can be refined by stable isotope studies
(e.g. Oberhänsli and Allen 1987; Talbot and Kelts
1989; Marshall 1992; Gat 1995). The heavy 18 0 in
authogenic carbonate and shells of organisms records
the isotopic composition of the paleolake water and,
in closed systems, its state of evaporation. In open
systems, the isotopic signature of lake carbonate reflects to a large degree the properties of river inflow
and thus the climate and altitude of the drainage area.
The primary isotope signal can be obscured by the
effects of diagenetic alteration, especially that of the
oxygen isotopes. For all these reasons, oxygen isotope values are sometimes difficult to interpret.
Relative enrichment of the heavy carbon isotope
13C indicates that plant material was strongly affected
by bacterial activity and that the lake floor possibly
emerged. Isotope studies also reveal that primary lake