346
Chapter 7 Sequences, Minor Cycles, and Event Stratigraphy
for example, moved up and down by tens of meters
during the Holocene, i.e. within time periods as short
as some tens to hundreds or thousand years. Most pronounced are fluctuations during glacial-interglacial
cycles controlled by orbital parameters (Milankovitch
frequency band ranging from 20 ka to 400 ka; Sect.
7.9.2). They caused drastic changes in both the levels
(up to some 100 m) and depositional environments of
many lakes.
(2) Large lakes in lowlands with lirnited terrigenous
sediment supply which may occasionally be influenced, directly or indirectly by sea-Ievel changes. Dependent on the clirnate, the hydrological regime of
these lakes tends to vary between open and closed, and
lake-level fluctuations have a low amplitude. Hence
they develop relatively thin sequences representing
time spans ranging from those of 3rd order sequences
ofthe marine realm to high-frequency cyclic phenomena. From the ancient record, a considerable number of
large, sediment-starved, long-persisting lake systems
are known. Their average sedimentation rates are much
slower than those found in most of the Quatemary
lakes. It may be possible to correlate some of these
ancient lake sequences with coeval marine sequences.
Examples of Lake level Fluctuations
(Late Pleistocene and Holocene)
Holocene lake levels in the Dead Sea fluctuated by some tens
of meters, Pleistocene levels by 200-300 m (Niemi et al.
1997). Observed historic lake level variations in the Great
Sa1t Lake and some 1akes in Asia amounted to > 10m (summary in Yan 1999). Modem hypersaline 1akes ("salt 1akes")
and playas in regions of arid to semiarid c1imates experienced cooler, wetter conditions during glacia1 periods or
during the transitions from glacia1s to interg1acia1s. Many
c10sed lake systems were transformed into open systems and
vice versa (Sect. 2.5). In the 1atter case, deposition of carbonates and evaporites was 1argely replaced by that of
si1icic1astic material. The thicknesses of such Pleistocene
clastic-evaporite cycles depend on the sedimentation rate in
the lake. One sequence or cyc1e may reach tens to hundreds
of meters in thickness. A prominent example is Death Valley
in Califomia where, in a permanently c10sed basin, two
dry/wet cyc1es accumulated during the past 200 ka, each
about 90 m in thickness (Lowenstein et al. 1999).
Lakes in regions of present -day subtropical climate may
have reacted in a different way. The late Pleistocene levels of
some of the East African lakes stood 200 m lower than the
present-day levels (summary in Shan1ey and McCabe 1994).
They brought about deep1y incised valleys, lowstand deltas
or fan deltas (on steep slopes), and evaporite deposition in
the basin center (Fig. 7.30b). Highstand deposits of deep
lakes, such as those of some young rift basins, may be characterized by slope failures, subaqueous talus on steep slopes,
deep channelized fans and turbidites in the basin center,
apart from hemipelagic muds.
In regions of temperate humid c1imates, the post-g1acia1
sediment successions known from many glacier-shaped lakes
along the margins of mountain ranges frequently display a
half-cycle sequence consisting of c1astic sediments followed
by carbonates (seekreide) and organic-rich muds.
Climate-Controlled Lacustrine Sedimentary Cycles
Cyclic sediments have been found in many ancient
lake basins. Whether or not they were controlled by
lake level fluctuations is often not clear because their
marginal sediments have been eroded. In the following
examples, lake-level fluctuations are only partially
documented. Principally, cyclic sequences of lakes
could originate solely from variations in clirnate. However, it is likely, at least for closed lake basins, that climate change not only affected sediment influx and
depositional environment, but also the lake levels.
Large lake basins display moderate sedimentation rates
and sequences or cycles of relatively long duration.
Ancient salt lakes of lirnited size, which had large
drainage areas, tended to become rapidly filled. Several of them show an excellent cyclicity at different
scales.
Some of the Triassic rift basins associated with the breakup
of Pangea (Sect. 12.3), for example the Newark rift in the
eastem United States, existed over aperiod of 30 to 40 Ma
(Smoot 1991). Their sediment fills inc1ude various lake deposits ofboth perennial and ephemera11ake systems (cf. Sect.
2.5). Dry phases were characterized by playa mud flats (desiccation.cracks), whereas wet phases led to larninated calcareous black shales (cf. Fig. 7AOa; van Houten 1964; Olsen
1990; Fischer 1991). In the so-called Newark Supergroup,
the mean thickness ofthe basic -20 ka-cyc1e is 5 to 6 m (sedimentation rate about 0.3 mlka); in the basin center, the cyc1es may contain some evaporites.
Recently, the total Newark basin fill of 1ate Triassic to
ear1y Jurassic age was again investigated by aseries of seven
overlapping drill holes. These revea1ed a continuous core
section of 4660 m in thickness (Olsen et al. 1998). Ofthese,
3700 m consist ofuninterrupted grey, black and reddish lake
sediments representing a time span of 24 Ma. This sequence
cou1d be subdivided into 53 large cyc1es (members of socalled McLaughlin cyc1es) of an average thickness of 70 m
and aperiod of 413 ka. Each of these members contains the
shorter 100 ka and 23 ka cyc1es. The lake level varied by up
to 200 m. The lacustrine cyc1es and their hierarchy could be
traced over the entire rift basin (>100 km). The cyc1icity is
also present in the upper reddish part of the 1acustrine basin
fill.
Another one of these rift basins (Richmond basin) was
controlled by more humid c1imate. Hs sedimentary succession contains coals and displays little evidence offalling dry.
Instead, the large downward shift of prograding deltas and
deep channels indicate that the basin must have been deep
during considerable time intervals.
A well-exposed example ofupper Triassic to early Jurassic (Rhaethian-Sinemurian) lake sediments (400 m to more
than 1000 m in thickness) has been described from East
Greenland (Dam and Surlyk 1993; Dam et al. 1995). The
fault-bounded lake basin is associated with rifting prior to
the opening of the North Atlantic. During the Triassic, the
environmental conditions changed from semi-arid to humid,
Chapter 7 Sequences, Minor Cycles, and Event Stratigraphy
for example, moved up and down by tens of meters
during the Holocene, i.e. within time periods as short
as some tens to hundreds or thousand years. Most pronounced are fluctuations during glacial-interglacial
cycles controlled by orbital parameters (Milankovitch
frequency band ranging from 20 ka to 400 ka; Sect.
7.9.2). They caused drastic changes in both the levels
(up to some 100 m) and depositional environments of
many lakes.
(2) Large lakes in lowlands with lirnited terrigenous
sediment supply which may occasionally be influenced, directly or indirectly by sea-Ievel changes. Dependent on the clirnate, the hydrological regime of
these lakes tends to vary between open and closed, and
lake-level fluctuations have a low amplitude. Hence
they develop relatively thin sequences representing
time spans ranging from those of 3rd order sequences
ofthe marine realm to high-frequency cyclic phenomena. From the ancient record, a considerable number of
large, sediment-starved, long-persisting lake systems
are known. Their average sedimentation rates are much
slower than those found in most of the Quatemary
lakes. It may be possible to correlate some of these
ancient lake sequences with coeval marine sequences.
Examples of Lake level Fluctuations
(Late Pleistocene and Holocene)
Holocene lake levels in the Dead Sea fluctuated by some tens
of meters, Pleistocene levels by 200-300 m (Niemi et al.
1997). Observed historic lake level variations in the Great
Sa1t Lake and some 1akes in Asia amounted to > 10m (summary in Yan 1999). Modem hypersaline 1akes ("salt 1akes")
and playas in regions of arid to semiarid c1imates experienced cooler, wetter conditions during glacia1 periods or
during the transitions from glacia1s to interg1acia1s. Many
c10sed lake systems were transformed into open systems and
vice versa (Sect. 2.5). In the 1atter case, deposition of carbonates and evaporites was 1argely replaced by that of
si1icic1astic material. The thicknesses of such Pleistocene
clastic-evaporite cycles depend on the sedimentation rate in
the lake. One sequence or cyc1e may reach tens to hundreds
of meters in thickness. A prominent example is Death Valley
in Califomia where, in a permanently c10sed basin, two
dry/wet cyc1es accumulated during the past 200 ka, each
about 90 m in thickness (Lowenstein et al. 1999).
Lakes in regions of present -day subtropical climate may
have reacted in a different way. The late Pleistocene levels of
some of the East African lakes stood 200 m lower than the
present-day levels (summary in Shan1ey and McCabe 1994).
They brought about deep1y incised valleys, lowstand deltas
or fan deltas (on steep slopes), and evaporite deposition in
the basin center (Fig. 7.30b). Highstand deposits of deep
lakes, such as those of some young rift basins, may be characterized by slope failures, subaqueous talus on steep slopes,
deep channelized fans and turbidites in the basin center,
apart from hemipelagic muds.
In regions of temperate humid c1imates, the post-g1acia1
sediment successions known from many glacier-shaped lakes
along the margins of mountain ranges frequently display a
half-cycle sequence consisting of c1astic sediments followed
by carbonates (seekreide) and organic-rich muds.
Climate-Controlled Lacustrine Sedimentary Cycles
Cyclic sediments have been found in many ancient
lake basins. Whether or not they were controlled by
lake level fluctuations is often not clear because their
marginal sediments have been eroded. In the following
examples, lake-level fluctuations are only partially
documented. Principally, cyclic sequences of lakes
could originate solely from variations in clirnate. However, it is likely, at least for closed lake basins, that climate change not only affected sediment influx and
depositional environment, but also the lake levels.
Large lake basins display moderate sedimentation rates
and sequences or cycles of relatively long duration.
Ancient salt lakes of lirnited size, which had large
drainage areas, tended to become rapidly filled. Several of them show an excellent cyclicity at different
scales.
Some of the Triassic rift basins associated with the breakup
of Pangea (Sect. 12.3), for example the Newark rift in the
eastem United States, existed over aperiod of 30 to 40 Ma
(Smoot 1991). Their sediment fills inc1ude various lake deposits ofboth perennial and ephemera11ake systems (cf. Sect.
2.5). Dry phases were characterized by playa mud flats (desiccation.cracks), whereas wet phases led to larninated calcareous black shales (cf. Fig. 7AOa; van Houten 1964; Olsen
1990; Fischer 1991). In the so-called Newark Supergroup,
the mean thickness ofthe basic -20 ka-cyc1e is 5 to 6 m (sedimentation rate about 0.3 mlka); in the basin center, the cyc1es may contain some evaporites.
Recently, the total Newark basin fill of 1ate Triassic to
ear1y Jurassic age was again investigated by aseries of seven
overlapping drill holes. These revea1ed a continuous core
section of 4660 m in thickness (Olsen et al. 1998). Ofthese,
3700 m consist ofuninterrupted grey, black and reddish lake
sediments representing a time span of 24 Ma. This sequence
cou1d be subdivided into 53 large cyc1es (members of socalled McLaughlin cyc1es) of an average thickness of 70 m
and aperiod of 413 ka. Each of these members contains the
shorter 100 ka and 23 ka cyc1es. The lake level varied by up
to 200 m. The lacustrine cyc1es and their hierarchy could be
traced over the entire rift basin (>100 km). The cyc1icity is
also present in the upper reddish part of the 1acustrine basin
fill.
Another one of these rift basins (Richmond basin) was
controlled by more humid c1imate. Hs sedimentary succession contains coals and displays little evidence offalling dry.
Instead, the large downward shift of prograding deltas and
deep channels indicate that the basin must have been deep
during considerable time intervals.
A well-exposed example ofupper Triassic to early Jurassic (Rhaethian-Sinemurian) lake sediments (400 m to more
than 1000 m in thickness) has been described from East
Greenland (Dam and Surlyk 1993; Dam et al. 1995). The
fault-bounded lake basin is associated with rifting prior to
the opening of the North Atlantic. During the Triassic, the
environmental conditions changed from semi-arid to humid,