7.6 Transitional Systems
different maturity may be used as means to reconstruct
the history of landscape evolution. In unlithified alluvial deposits, the lowered sea level may lead to a wide
incised valley where the river system is transformed
from a meandering to a braided style.
Valley incision on land and on continental shelves
can be significantly modified by glaciers creating wide
and often also deep U-shaped valleys and fjords. The
Pleistocene glacial periods have left behind manyexamples of this type of valley along and off the coasts
of formerly glaciated regions (cf. Sect. 2.1). Finally, it
should be mentioned that many incised valleys fi1led
by marine, coastal and fluvial sediments solely originate from tectonic uplift or from both uplift and
eustatic sea-level changes (e.g. on active continental
margins; cf. Sect. 5.4).
Examples of these valley types have been described from
many regions, e.g. northem Spain (Marzo et al. 1998). The
spectacular Eocene fanglomerates ofthe Montserrat in northeastem Spain seem to have been controlled bytectonics (and
possibly c1imate change) in their source area (an overthrust
belt) as well as by sea-level fluctuations in distal reaches
(Bums et al. 1997).
A complex example of a tectonically controlled halfgraben fill affected by several phases of sea leve1-induced
valley incision has been reported from the Eocene of Seymour Island, Antarctic Peninsula (Porebski 1995). The fill of
this basin is 6 km wide and 0.4 km thick and can be subdivided into several discrete sequences separated by erosional
unconformities. The facies tracts inc1ude deltaic, tidal and
estuarine sediments and show distinct maximum flooding
surfaces.
7.6.2 Coal in Incised Valleys and Flooded Coastal
Plains
Coal in Incised Valleys
As mentioned above, manyancient incised valleys provided habitats for the formation of coal (cf. Sect. 14.4).
A general model of the filling of such valleys is shown
in Fig. 7.28a (see e.g. Martinsen 1994; Richards 1996).
This facies model is mainly based on examples from
foreland basins with high to moderate sediment supply.
Valley cutting during relative sea-level fall may trigger
some sediment gravity movements and leave behind
relics of terrace deposits; on the shoulders and, to a
minor degree, on the valley flanks soils can form.
During the late lowstand the lowermost portion of
the valley is commonly fi1led with deposits of braided
rivers. With the onset of transgression (initial
transgressive surface), the accommodation space for
sediment aggradation increases and the gradient ofthe
river decreases. As a result, the bed-Ioad channels of
the formerly braided rivers (Fig. 7 .28al) tend to evolve
into less amalgamated and more sinuous systems. The
medium-gradient mixed-load channel become more
isolated within fine-grained floodplain deposits. Fur339
ther upsection, such systems may be replaced by muddominated anastomosing channel-floodplain conditions. Coal searns developed during this phase of valley filling are normally thin and not very extensive.
At the transition from TST to HST, wide areas outside of the valleys are flooded. This is the time period
in which thick and extensive coal deposits can form,
either somewhat below or above the maximum flooding surface (MFS). In relation to the position of the
coastline and sediment sourees, the sedimentary facies
of this time interval vary from interdistributary lakes
and swamps to deltaic, estuarine, tidal (ineluding tidalinfluenced channels) to shallow-marine conditions.
When the relative sea level starts to fall again during
the late highstand phase, the sediment accommodation
space is reduced and amalgamation of fluvial channels
and their connectivity increase. In this way, channel
units become more and more truncated and form sheet
sandstone bodies. Coal searns again become thin and
limited in their lateral extent. Continued relative sealevel fall in the subsequent lowstand phase will cut
new valleys into the emerging former wide depositional area (Fig. 7.28b).
In the idealized cross-section ofFig. 7.28b, the valley was
cut down to the top of the previous marine limestones. All
the coal 8eams formed during transgression (TST), but the
most wideiy extended seam evolved in the uppermost part of
TST and is partially overlain by a marine limestone. The
transition from TST to HST is marked by shelly limestones
and a horizon with phosphorite nodules (condensed section).
Incised valley-fill sequences have been described from
many regions and time periods. Examples are the Carboniferous cyc10thems of Kansas and the Appalachians, United
States (Archer et al. 1994; Chesnut 1994) which differ in
their nature. The principal relationship between the accommodation rate and the rate at which peat can grow is addressed by Bohacs and Suter (1997).
The major coal seam ofFigure 7.28b is affected by a
second period of valley incision. This phenomenon is
common when coal searns occur elose to the maximum
flooding surface. In these cases, the frequency and amplitude of the eustatic sea-level change was high in
comparison to a moderate rate of subsidence (cf. Fig.
7.6c, where only the lowermost part of TST is preserved). However, cyelothems of the Appalchians,
consisting of silicielastics and coal, deviate from the
previous models (Fig. 7 .28c). Here, valley incision
normally does not reach the underlying coal deposited
at the transition from TST to HST. The mean duration
of one Appalachian cyele is about 0.4 Ma (the longest
time interval ofthe Milankovitch band). As in the previous models, the section above the coal seam is
coarsening-upward (HST), whereas the valley fill is
fining-upward (TST).
In summary, coal searns in incised valleys rnainly
form in the late TST and at the transition from TST to
HST. Their full or partial preservation depends on the
accommodation space which is left after the subse-
different maturity may be used as means to reconstruct
the history of landscape evolution. In unlithified alluvial deposits, the lowered sea level may lead to a wide
incised valley where the river system is transformed
from a meandering to a braided style.
Valley incision on land and on continental shelves
can be significantly modified by glaciers creating wide
and often also deep U-shaped valleys and fjords. The
Pleistocene glacial periods have left behind manyexamples of this type of valley along and off the coasts
of formerly glaciated regions (cf. Sect. 2.1). Finally, it
should be mentioned that many incised valleys fi1led
by marine, coastal and fluvial sediments solely originate from tectonic uplift or from both uplift and
eustatic sea-level changes (e.g. on active continental
margins; cf. Sect. 5.4).
Examples of these valley types have been described from
many regions, e.g. northem Spain (Marzo et al. 1998). The
spectacular Eocene fanglomerates ofthe Montserrat in northeastem Spain seem to have been controlled bytectonics (and
possibly c1imate change) in their source area (an overthrust
belt) as well as by sea-level fluctuations in distal reaches
(Bums et al. 1997).
A complex example of a tectonically controlled halfgraben fill affected by several phases of sea leve1-induced
valley incision has been reported from the Eocene of Seymour Island, Antarctic Peninsula (Porebski 1995). The fill of
this basin is 6 km wide and 0.4 km thick and can be subdivided into several discrete sequences separated by erosional
unconformities. The facies tracts inc1ude deltaic, tidal and
estuarine sediments and show distinct maximum flooding
surfaces.
7.6.2 Coal in Incised Valleys and Flooded Coastal
Plains
Coal in Incised Valleys
As mentioned above, manyancient incised valleys provided habitats for the formation of coal (cf. Sect. 14.4).
A general model of the filling of such valleys is shown
in Fig. 7.28a (see e.g. Martinsen 1994; Richards 1996).
This facies model is mainly based on examples from
foreland basins with high to moderate sediment supply.
Valley cutting during relative sea-level fall may trigger
some sediment gravity movements and leave behind
relics of terrace deposits; on the shoulders and, to a
minor degree, on the valley flanks soils can form.
During the late lowstand the lowermost portion of
the valley is commonly fi1led with deposits of braided
rivers. With the onset of transgression (initial
transgressive surface), the accommodation space for
sediment aggradation increases and the gradient ofthe
river decreases. As a result, the bed-Ioad channels of
the formerly braided rivers (Fig. 7 .28al) tend to evolve
into less amalgamated and more sinuous systems. The
medium-gradient mixed-load channel become more
isolated within fine-grained floodplain deposits. Fur339
ther upsection, such systems may be replaced by muddominated anastomosing channel-floodplain conditions. Coal searns developed during this phase of valley filling are normally thin and not very extensive.
At the transition from TST to HST, wide areas outside of the valleys are flooded. This is the time period
in which thick and extensive coal deposits can form,
either somewhat below or above the maximum flooding surface (MFS). In relation to the position of the
coastline and sediment sourees, the sedimentary facies
of this time interval vary from interdistributary lakes
and swamps to deltaic, estuarine, tidal (ineluding tidalinfluenced channels) to shallow-marine conditions.
When the relative sea level starts to fall again during
the late highstand phase, the sediment accommodation
space is reduced and amalgamation of fluvial channels
and their connectivity increase. In this way, channel
units become more and more truncated and form sheet
sandstone bodies. Coal searns again become thin and
limited in their lateral extent. Continued relative sealevel fall in the subsequent lowstand phase will cut
new valleys into the emerging former wide depositional area (Fig. 7.28b).
In the idealized cross-section ofFig. 7.28b, the valley was
cut down to the top of the previous marine limestones. All
the coal 8eams formed during transgression (TST), but the
most wideiy extended seam evolved in the uppermost part of
TST and is partially overlain by a marine limestone. The
transition from TST to HST is marked by shelly limestones
and a horizon with phosphorite nodules (condensed section).
Incised valley-fill sequences have been described from
many regions and time periods. Examples are the Carboniferous cyc10thems of Kansas and the Appalachians, United
States (Archer et al. 1994; Chesnut 1994) which differ in
their nature. The principal relationship between the accommodation rate and the rate at which peat can grow is addressed by Bohacs and Suter (1997).
The major coal seam ofFigure 7.28b is affected by a
second period of valley incision. This phenomenon is
common when coal searns occur elose to the maximum
flooding surface. In these cases, the frequency and amplitude of the eustatic sea-level change was high in
comparison to a moderate rate of subsidence (cf. Fig.
7.6c, where only the lowermost part of TST is preserved). However, cyelothems of the Appalchians,
consisting of silicielastics and coal, deviate from the
previous models (Fig. 7 .28c). Here, valley incision
normally does not reach the underlying coal deposited
at the transition from TST to HST. The mean duration
of one Appalachian cyele is about 0.4 Ma (the longest
time interval ofthe Milankovitch band). As in the previous models, the section above the coal seam is
coarsening-upward (HST), whereas the valley fill is
fining-upward (TST).
In summary, coal searns in incised valleys rnainly
form in the late TST and at the transition from TST to
HST. Their full or partial preservation depends on the
accommodation space which is left after the subse-