76
by tectonic movements. The present-day glaciershaped lakes therefore provide poor analogs for the
interpretation of many ancient lake systems. This is
the reason why other lake types in ice-free regions
are dealt with in more detail than, e.g., perialpine
lakes. Furthermore, only few modem lakes have an
areal extent and accumulate sediment thicknesses
similar to those of prominent fossil counterparts.
Lakes are often referred to as "clearing basins" of
rivers. The total bedload and most of the suspended
load of entering rivers settle out in the lake. Lakes
and their corresponding drainage areas therefore can
be regarded as sedimentologically closed denudationaccumulation systems (cf. Sect. 11.2) even when they
are hydrologically open. Lakes of limited size and
high influx of detrital clastics are short-lived,
whereas large and deep lakes, particularly those in
areas of long-Iasting crustal subsidence and low
terrigenous influx, can pers ist for long geological
time periods. It is this latter type of lake which best
displays the sensitivity of lacustrine sediments to
changing climatic conditions.
2.5.2 General Processes in Various Lake
Systems (Overview)
The great diversity of lake basins and their sedimentary fill makes it difficult to summarize our knowledge in a simple classification scheme. Apart from
the factors mentioned above, we have to consider
physical, hydrological, biological, and chemical processes which play a great part, particularly in longpersisting lakes. In contrast to the open sea, waves
and wind-driven currents are generally much less
important in lakes for the distribution and reworking
of sediments. These processes operate to some extent
along the lake shores but normally do not affect the
lake bottom, unless the lake is very shallow and
large. True tidal currents are absent, but wind stress
may repeatedly drive water against a shore and create
"wind-tidal flats".
For these reasons, lake waters tend to become
stratified either permanently or seasonally (Fig.
2.28b I). Permanent stratification is characteristic of
tropical lakes (temperature stratification, oligomictic
lakes) and lakes in which the bottom water is more
saline than surface water (meromictic lakes). In both
cases, the bottom water (hypolimnion) can become
completely stagnant and be depleted in oxygen and
nutrients, which leads to restricted benthic life and
the preservation of organic matter produced in the
near-surface water (epilimnion). If such stratified
conditions are maintained for a long time period, i.e.,
for at least several thousands of years, laminated
muds rich in organic matter (sapropel) can accumulate (see below).
Chapter 2 Continental Sediments
In regions 0: temperate climate, the surficial lake
water is warmer during the summer and less den se
than the deeper water; hence, lakes with limited inflow and outflow are stratified. However, during the
winter the surface water cools and reaches the same
or a higher density as the bottom water. Consequently, the total lake water body can be tumed over
once or two times a year and become mixed
(monomictic or dimictic lakes, Fig. 2.28a). In this
case, a permanently stagnant hypolirnnion with fully
anoxic conditions cannot develop. Similarly, a major
river crossing a lake will cause underflow (Fig.
2.28a) if its water is cool and comparatively dense.
Then the total lake water body including the
hypolirnnion is weIl supplied with oxygen and enables a fairly high seasonal biogenic production (e.g.,
diatoms, different types of algae, etc).
Such conditions are fairly common in regions of
humid climate where permanent, sufficient inflow of
river water keeps the lake filled with freshwater and
thus maintains a hydrologically open system. In this
case, the entire depositional system is more or less
stable, apart from seasonal variations generating
varves (see below). Most of these open lakes are
dominated proximally by river-derived clastic sediments and distally by rnixtures of fine-grained
clastics, carbonate, and biogenic silica.
Hydrologically closed lake systems have no surface outflow (Eg. 2.28b), but some of them are leaky
and infiltrate into groundwater. Completely closed or
semi-closed lakes are common in regions of warm,
semi-arid climate. If evaporation of lake water is
ab out equal to inflow, a permanent water body can be
maintained (perennial lakes, Fig. 2.28bl and b2).
However, as a result of minor climatic variations in
the range of 10 to some 100 years, the water level of
such lakes may fluctuate considerably and affect a
wide beach and near-shore zone (sand flats or mud
flats, Fig. 2.28b and bl) by repeated emergence and
inundation.
In the late Pleistocene, the water levels of Great Salt Lake
in North America, Dead Sea, and some lakes in the East
African Rift zone varied by several hundreds of meters.
Even in the Holocene lake fluctuations of 10m and more
are common (Currey 1990; Niemi et al. 1997; Scholz and
Rosendahl (1990).
During periods of high water level, the chernistry of
lake water may change markedly, either by dilution
of highly mineralized water or re-dissolution of salts
precipitated during emergence in the near-shore
zone. Deeper lakes tend to evolve stable stratified
conditions with oxygen-depleted bottom water.
During longer time periods (thousands of years)
the concentration of highly soluble species in the
lake water tends to increase and finally to lead to the
precipitation of salts. Ephemeral or playa lakes (in-
by tectonic movements. The present-day glaciershaped lakes therefore provide poor analogs for the
interpretation of many ancient lake systems. This is
the reason why other lake types in ice-free regions
are dealt with in more detail than, e.g., perialpine
lakes. Furthermore, only few modem lakes have an
areal extent and accumulate sediment thicknesses
similar to those of prominent fossil counterparts.
Lakes are often referred to as "clearing basins" of
rivers. The total bedload and most of the suspended
load of entering rivers settle out in the lake. Lakes
and their corresponding drainage areas therefore can
be regarded as sedimentologically closed denudationaccumulation systems (cf. Sect. 11.2) even when they
are hydrologically open. Lakes of limited size and
high influx of detrital clastics are short-lived,
whereas large and deep lakes, particularly those in
areas of long-Iasting crustal subsidence and low
terrigenous influx, can pers ist for long geological
time periods. It is this latter type of lake which best
displays the sensitivity of lacustrine sediments to
changing climatic conditions.
2.5.2 General Processes in Various Lake
Systems (Overview)
The great diversity of lake basins and their sedimentary fill makes it difficult to summarize our knowledge in a simple classification scheme. Apart from
the factors mentioned above, we have to consider
physical, hydrological, biological, and chemical processes which play a great part, particularly in longpersisting lakes. In contrast to the open sea, waves
and wind-driven currents are generally much less
important in lakes for the distribution and reworking
of sediments. These processes operate to some extent
along the lake shores but normally do not affect the
lake bottom, unless the lake is very shallow and
large. True tidal currents are absent, but wind stress
may repeatedly drive water against a shore and create
"wind-tidal flats".
For these reasons, lake waters tend to become
stratified either permanently or seasonally (Fig.
2.28b I). Permanent stratification is characteristic of
tropical lakes (temperature stratification, oligomictic
lakes) and lakes in which the bottom water is more
saline than surface water (meromictic lakes). In both
cases, the bottom water (hypolimnion) can become
completely stagnant and be depleted in oxygen and
nutrients, which leads to restricted benthic life and
the preservation of organic matter produced in the
near-surface water (epilimnion). If such stratified
conditions are maintained for a long time period, i.e.,
for at least several thousands of years, laminated
muds rich in organic matter (sapropel) can accumulate (see below).
Chapter 2 Continental Sediments
In regions 0: temperate climate, the surficial lake
water is warmer during the summer and less den se
than the deeper water; hence, lakes with limited inflow and outflow are stratified. However, during the
winter the surface water cools and reaches the same
or a higher density as the bottom water. Consequently, the total lake water body can be tumed over
once or two times a year and become mixed
(monomictic or dimictic lakes, Fig. 2.28a). In this
case, a permanently stagnant hypolirnnion with fully
anoxic conditions cannot develop. Similarly, a major
river crossing a lake will cause underflow (Fig.
2.28a) if its water is cool and comparatively dense.
Then the total lake water body including the
hypolirnnion is weIl supplied with oxygen and enables a fairly high seasonal biogenic production (e.g.,
diatoms, different types of algae, etc).
Such conditions are fairly common in regions of
humid climate where permanent, sufficient inflow of
river water keeps the lake filled with freshwater and
thus maintains a hydrologically open system. In this
case, the entire depositional system is more or less
stable, apart from seasonal variations generating
varves (see below). Most of these open lakes are
dominated proximally by river-derived clastic sediments and distally by rnixtures of fine-grained
clastics, carbonate, and biogenic silica.
Hydrologically closed lake systems have no surface outflow (Eg. 2.28b), but some of them are leaky
and infiltrate into groundwater. Completely closed or
semi-closed lakes are common in regions of warm,
semi-arid climate. If evaporation of lake water is
ab out equal to inflow, a permanent water body can be
maintained (perennial lakes, Fig. 2.28bl and b2).
However, as a result of minor climatic variations in
the range of 10 to some 100 years, the water level of
such lakes may fluctuate considerably and affect a
wide beach and near-shore zone (sand flats or mud
flats, Fig. 2.28b and bl) by repeated emergence and
inundation.
In the late Pleistocene, the water levels of Great Salt Lake
in North America, Dead Sea, and some lakes in the East
African Rift zone varied by several hundreds of meters.
Even in the Holocene lake fluctuations of 10m and more
are common (Currey 1990; Niemi et al. 1997; Scholz and
Rosendahl (1990).
During periods of high water level, the chernistry of
lake water may change markedly, either by dilution
of highly mineralized water or re-dissolution of salts
precipitated during emergence in the near-shore
zone. Deeper lakes tend to evolve stable stratified
conditions with oxygen-depleted bottom water.
During longer time periods (thousands of years)
the concentration of highly soluble species in the
lake water tends to increase and finally to lead to the
precipitation of salts. Ephemeral or playa lakes (in-