7.7 Continental Sequence Stratigraphy
345
7.6.5 Summary (Incised VaUeys, Coastal Plains, Coal)
- Valley incision during sea-level fall commonly
affects coastal plains and their hinterland, the
inner and middle shelf zones, and sometimes
also the shelfbreak and slope.
- Fluvial deposits at the base of the wide incised
channels are normally followed by marine and
lagoonal deposits of the transgressive systems
tract. These are often capped by coal seams and
marine carbonates representing the late TST and
early HST.
Interchannel areas experience erosion andJor
pedogenesis. Whether or not they become later
7.7 Continental Sequence Stratigraphy
Since some years, the general principles of marine sequence stratigraphy are also being applied to pure continental depositional systems which are not affected by
sea-level change. One group ofthese systems is associated with lakes, the water levels of which are mainly
controlled by tectonics and the climatic conditions in
their drainage areas. The other group comprises various fluvial systems largely controlled by the relief (tectonics) and climate in their upper reaches. In the case
of lakes, the lake level is commonly regarded as the
ultimate base level of the entering rivers (cf. Sect.
7.2.2). Fluvial systems filling a subsiding fluvial basin
with an inclined surface normally do not have a base
level which can be regarded as being fixed at a certain
location. Instead they are characterized by a river gradient representing the base level ofthe system (cf. Fig.
7.3a and Sect. 7.7.2).
Continental sequences which have formed independently from the sea do not show the same orders as
sea level-controlled sequences. The time periods of
these sequences also greatly vary, but they are often sI
Ma, i.e., in the range of the Milankovitch frequency
band. A clear distinction between the classical2nd and
3rd order sequences ofthe marine regime and the continental sequences, partially representing shorter cycles
(cf. Sect. 7.9, Cyclostratigraphy) is hardly possible.
7.7.1 Lacustrine Sequences and Cycles
General Aspects
Lacustrine sediments generally respond to lake-level
changes in a similar way as do marine depositional
systems to sea-level changes described above. Lakelevel fall leads to a downward shift of facies tracts,
whereas lake-level rise often causes alandward jump
flooded depends on the amplitude of the subsequent sea-Ievel rise and the rate of subsidence.
- When coastallowlands and lagoons are periodically flooded, they undergo drastic environmental changes. These are preserved in cyclothems
containing paleosols, coal, fluvial and marine
deposits.
- Paralic coal formation in swamps requires a
certain rate in the gain of accommodation space
as weIl as cutoff from terrestrial sediment supply.
of the deltaic and marginal lake facies. In addition,
lake sediments respond rapidly and markedly to various other environmental changes both in the lake areas
themselves (e.g. oxygen supply), as weIl as in their
drainage areas (Sect. 2.5). When lake sediments accumulate under stratified water conditions, their bedding
and varves often allow a very accurate lithostratigraphy and, in favorable cases, chronostratigraphy.
One can distinguish two types of lake systems:
(1) Lakes far away from the sea andJor located sufficiently high above sea level. Their rising and falling
lake levels are controlled by local factors and therefore
vary strongly from lake to lake. It has to be generally
assumed that these lake-level curves are both asymmetric (cf. Fig. 7.8) and very irregular. Hydrologically
open and closed lake basins behave in a different way.
- The water level of hydrologically open lakes is a
function of the elevation of their outflow (groundwater
leakage is not considered here). This can be lowered
with time due to erosional downcutting andJor tectonic
subsidence or it may rise as a result of uplift (Fig.
7.30a). Tectonism seems to be the main factor controlling lacustrine depositional sequences in regions of
humid climate (e.g. Scholz et al. 1998). If tectonism is
involved the lake level changes relatively slowly, i.e.
some tens of meters within a time period of 0.1 to 1
Ma. Erosional downcutting may proceed faster. As a
result, entering rivers either cut deeper valleys and
their deltas are forced to prograde basinward or, with
rising level, the deltas tend to step back and cause increased aggradation upstream on their alluvial plains
(Fig. 7.30b). Independently from these long-term
trends, the lake sediments, particularly so in the lake
center, may displayakind of minor cyclicity resulting
from climatic changes (cf. Sect. 2.5).
- In contrast, the water levels of closed lake basins
often show high-frequency, but very irregular fluctuations (Fig. 7.30a). The water levels of modem lakes,
345
7.6.5 Summary (Incised VaUeys, Coastal Plains, Coal)
- Valley incision during sea-level fall commonly
affects coastal plains and their hinterland, the
inner and middle shelf zones, and sometimes
also the shelfbreak and slope.
- Fluvial deposits at the base of the wide incised
channels are normally followed by marine and
lagoonal deposits of the transgressive systems
tract. These are often capped by coal seams and
marine carbonates representing the late TST and
early HST.
Interchannel areas experience erosion andJor
pedogenesis. Whether or not they become later
7.7 Continental Sequence Stratigraphy
Since some years, the general principles of marine sequence stratigraphy are also being applied to pure continental depositional systems which are not affected by
sea-level change. One group ofthese systems is associated with lakes, the water levels of which are mainly
controlled by tectonics and the climatic conditions in
their drainage areas. The other group comprises various fluvial systems largely controlled by the relief (tectonics) and climate in their upper reaches. In the case
of lakes, the lake level is commonly regarded as the
ultimate base level of the entering rivers (cf. Sect.
7.2.2). Fluvial systems filling a subsiding fluvial basin
with an inclined surface normally do not have a base
level which can be regarded as being fixed at a certain
location. Instead they are characterized by a river gradient representing the base level ofthe system (cf. Fig.
7.3a and Sect. 7.7.2).
Continental sequences which have formed independently from the sea do not show the same orders as
sea level-controlled sequences. The time periods of
these sequences also greatly vary, but they are often sI
Ma, i.e., in the range of the Milankovitch frequency
band. A clear distinction between the classical2nd and
3rd order sequences ofthe marine regime and the continental sequences, partially representing shorter cycles
(cf. Sect. 7.9, Cyclostratigraphy) is hardly possible.
7.7.1 Lacustrine Sequences and Cycles
General Aspects
Lacustrine sediments generally respond to lake-level
changes in a similar way as do marine depositional
systems to sea-level changes described above. Lakelevel fall leads to a downward shift of facies tracts,
whereas lake-level rise often causes alandward jump
flooded depends on the amplitude of the subsequent sea-Ievel rise and the rate of subsidence.
- When coastallowlands and lagoons are periodically flooded, they undergo drastic environmental changes. These are preserved in cyclothems
containing paleosols, coal, fluvial and marine
deposits.
- Paralic coal formation in swamps requires a
certain rate in the gain of accommodation space
as weIl as cutoff from terrestrial sediment supply.
of the deltaic and marginal lake facies. In addition,
lake sediments respond rapidly and markedly to various other environmental changes both in the lake areas
themselves (e.g. oxygen supply), as weIl as in their
drainage areas (Sect. 2.5). When lake sediments accumulate under stratified water conditions, their bedding
and varves often allow a very accurate lithostratigraphy and, in favorable cases, chronostratigraphy.
One can distinguish two types of lake systems:
(1) Lakes far away from the sea andJor located sufficiently high above sea level. Their rising and falling
lake levels are controlled by local factors and therefore
vary strongly from lake to lake. It has to be generally
assumed that these lake-level curves are both asymmetric (cf. Fig. 7.8) and very irregular. Hydrologically
open and closed lake basins behave in a different way.
- The water level of hydrologically open lakes is a
function of the elevation of their outflow (groundwater
leakage is not considered here). This can be lowered
with time due to erosional downcutting andJor tectonic
subsidence or it may rise as a result of uplift (Fig.
7.30a). Tectonism seems to be the main factor controlling lacustrine depositional sequences in regions of
humid climate (e.g. Scholz et al. 1998). If tectonism is
involved the lake level changes relatively slowly, i.e.
some tens of meters within a time period of 0.1 to 1
Ma. Erosional downcutting may proceed faster. As a
result, entering rivers either cut deeper valleys and
their deltas are forced to prograde basinward or, with
rising level, the deltas tend to step back and cause increased aggradation upstream on their alluvial plains
(Fig. 7.30b). Independently from these long-term
trends, the lake sediments, particularly so in the lake
center, may displayakind of minor cyclicity resulting
from climatic changes (cf. Sect. 2.5).
- In contrast, the water levels of closed lake basins
often show high-frequency, but very irregular fluctuations (Fig. 7.30a). The water levels of modem lakes,