342
Chapter 7 Sequences, Minor Cycles, and Event Stratigraphy
An example for the response of widely extended
lowlands to glacio-eustatic sea-level changes of high
frequency and amplitude are Carboniferous cyclotheros ofthe shale-paleosol type. They occur in central
North America and are known from an area of about
500000 km 2 (Fig. 7.28d).
These cyelothems are characterized by limited terrigenous
sediment supply, a marine black shale horizon, limestones,
and paleosols. This type of cyelothem was described by several authors (summary in Riegel 1991) who pointed out that
they originate either from glacio-eustasy, or from tectonics
and autocyelic processes (e.g. delta switching). Figure 7.28d
exhibits a typical representative ofthese cyelothems in a simplified way, according to the revised interpretation by Heckel
(1994). This interpretation is mainly based on faunistic and
sedimentological evidence which was used to evaluate the
paleo-water depth ofthe black shale (around 100m) and two
marine limestone horizons below and on top of the black
shale. The grey to green, locally red paleosols are taken as
lower and upper sequence boundaries ofthe cycle; paleosols
may be replaced locally by coal searns.
The repeated change from a terrestrial to a marine
depositional environment of about 100 m in water depth at a
frequency in therange ofthe Milankovitch band (ca. 20 ka to
400 ka) canhardlybe explained bytectonism. Rather, steady
basin subsidence and limited terrestrial sediment input have
maintained an environment fluctuating between an extensive
coastallowland and shallow-sea area for a considerable time
period. The high-frequency variation in water depth must
have been caused by glacio-eustasy. Farther landward, i.e. in
a proximal position, the coal-bearing cyelothems become
more irregular due to incised valleys, erosional disconformities, and widespread paleosols (Heckel et al. 1998).
The approximately time-equivalent, but lithologically differing coal cyelothems in Texas, Illinois and the Appalchians
are rich in terrestrial detritus but poor in carbonates (cf. Fig.
7.28c). The possibility of correlating Permo-Carboniferous
cyelothems between non-foreland settings (Nevada, Utah)
and those of a foreland basin (southem Cordilleran region,
USA) exeludes a pure tectonic origin of the cyeles
(Dickinson et al. 1994). In the case of fore land basins, coeval
overthrusting (causing basin subsidence) and uplift in the
forebulge region can produce out-of-phase cycles between
the basin interior and outer basin margin. In addition, these
cyeles hardly record Milankovitch frequencies.
Permo-Carboniferous paralic coal cyelothems are also
known from other regions, e.g. North China (Liu and Ricken
1997). Here, in an epicontinental setting up to 22 cyeles of
the 3rd or 4th order have been recognized. The cyeles consist
of sandstones, shales, coal searns and marine limestones. The
coal searnsformed during TST and are directly overlain by
the limestones representing the late TST and early HST. During transgressions the coastline commonly migrated 200 to
600 km landward.
Fig. 7.28. al,a2 General models for the filling of
incised valleys (after Richards 1996, modified).
b Coal-bearing Carboniferous valley fills in Kansas
(after Archer et al. 1994, modified). c Idealized Appalachian coal cyclothems (after Chestnut 1994).
d Typical black shale-paleosol cyclothem of the CarThe Carboniferous coal cyelothems of the Ruhr Basin in
northwestem Germany are an exarnple for the interplay of
sea-level changes and the irregular growth of coastal plains
elose to marine deltas. In previous studies, these cyelothems
were thought to cover wide areas without significant lateral
change and to result solely from sea-level changes. However,
more detailed information on the facies architecture of these
proximal foreland basin strata has revealed their complex
nature (Süss 1996). Prograding and laterally switching
subdeltas in combination with subsidence and sediment compaction created interdistributary swamps and shallow
embayments favorable for the generation of various sorts of
peat. This autocyclic mechanism is responsible for the splitting and pinching out of coal seams and other facies types.
Overprint of the deltaic environment by high-frequency (100
to 400 ka) sea-level changes of large amplitude created
shallowing-upward parasequences, ranging from 60 to 100 m
in thickness. The lower part ofthese parasequences is characterized by marine flooding surfaces, the upper part by
paleosols indicating continued regression of the sea.
7.6.3 Coastal Lagoons and Lowland LakeslPlayas
Specific marine flooding events mayaiso affect large
lagoons or lake basins normally separated from the sea
(Fig. 7 .29b). Then either freshwater and brackish water
or high-salinity brines are intermittently replaced by
environments approaching or reaching normal marine
conditions and their corresponding biota. Individual
layers or thicker strata reflecting these conditions indicate the "maximum flooding surface" of such events.
Relatively short marine ingress ions into long-persisting continental basins, in particular intracratonic
basins, are known from many countries and epochs.
The ingressions may leave behind marine shales and
carbonate horizons or, in some cases, thick gypsum
layers which can hardly be explained by the dissolved
load of entering rivers poor in sulfate. Flooding of coal
swamps and the deposition of marine shales have been
mentioned above. The influence of the sea-Ievel
highstand may be overprinted by climate change leading to greatly variable depositional systems.
The Germanic facies province of the Triassic in Europe
mainly consists of continental red beds altemating with some
marine beds caused by a few prominent and several minor
marine flooding periods (Sect. 12.3.2).
These sequences have been recently interpreted in terms
ofsequence stratigraphy (e.g. Aigner and Bachmann 1992).
At the transition from the Middle Triassic (Muschelkalk) to
the Upper Triassic (Keuper) minor marine flooding events
have affected not only coastal plain deposits (dolomite overlain by shale with coal) but also generated coastal sabkhas
precipitating subtidal to supratidal gypsum (Aigner 1998) .
•
boniferous in North America (after Heckel 1994,
simplified). e Lower Miocene paleo-valley incised
along distal margin of northem Alpine fore land basin
into marine and freshwater molasse deposits, southem Germany (after Buchner et al. 1996, modified).
For further explanation see text
Chapter 7 Sequences, Minor Cycles, and Event Stratigraphy
An example for the response of widely extended
lowlands to glacio-eustatic sea-level changes of high
frequency and amplitude are Carboniferous cyclotheros ofthe shale-paleosol type. They occur in central
North America and are known from an area of about
500000 km 2 (Fig. 7.28d).
These cyelothems are characterized by limited terrigenous
sediment supply, a marine black shale horizon, limestones,
and paleosols. This type of cyelothem was described by several authors (summary in Riegel 1991) who pointed out that
they originate either from glacio-eustasy, or from tectonics
and autocyelic processes (e.g. delta switching). Figure 7.28d
exhibits a typical representative ofthese cyelothems in a simplified way, according to the revised interpretation by Heckel
(1994). This interpretation is mainly based on faunistic and
sedimentological evidence which was used to evaluate the
paleo-water depth ofthe black shale (around 100m) and two
marine limestone horizons below and on top of the black
shale. The grey to green, locally red paleosols are taken as
lower and upper sequence boundaries ofthe cycle; paleosols
may be replaced locally by coal searns.
The repeated change from a terrestrial to a marine
depositional environment of about 100 m in water depth at a
frequency in therange ofthe Milankovitch band (ca. 20 ka to
400 ka) canhardlybe explained bytectonism. Rather, steady
basin subsidence and limited terrestrial sediment input have
maintained an environment fluctuating between an extensive
coastallowland and shallow-sea area for a considerable time
period. The high-frequency variation in water depth must
have been caused by glacio-eustasy. Farther landward, i.e. in
a proximal position, the coal-bearing cyelothems become
more irregular due to incised valleys, erosional disconformities, and widespread paleosols (Heckel et al. 1998).
The approximately time-equivalent, but lithologically differing coal cyelothems in Texas, Illinois and the Appalchians
are rich in terrestrial detritus but poor in carbonates (cf. Fig.
7.28c). The possibility of correlating Permo-Carboniferous
cyelothems between non-foreland settings (Nevada, Utah)
and those of a foreland basin (southem Cordilleran region,
USA) exeludes a pure tectonic origin of the cyeles
(Dickinson et al. 1994). In the case of fore land basins, coeval
overthrusting (causing basin subsidence) and uplift in the
forebulge region can produce out-of-phase cycles between
the basin interior and outer basin margin. In addition, these
cyeles hardly record Milankovitch frequencies.
Permo-Carboniferous paralic coal cyelothems are also
known from other regions, e.g. North China (Liu and Ricken
1997). Here, in an epicontinental setting up to 22 cyeles of
the 3rd or 4th order have been recognized. The cyeles consist
of sandstones, shales, coal searns and marine limestones. The
coal searnsformed during TST and are directly overlain by
the limestones representing the late TST and early HST. During transgressions the coastline commonly migrated 200 to
600 km landward.
Fig. 7.28. al,a2 General models for the filling of
incised valleys (after Richards 1996, modified).
b Coal-bearing Carboniferous valley fills in Kansas
(after Archer et al. 1994, modified). c Idealized Appalachian coal cyclothems (after Chestnut 1994).
d Typical black shale-paleosol cyclothem of the CarThe Carboniferous coal cyelothems of the Ruhr Basin in
northwestem Germany are an exarnple for the interplay of
sea-level changes and the irregular growth of coastal plains
elose to marine deltas. In previous studies, these cyelothems
were thought to cover wide areas without significant lateral
change and to result solely from sea-level changes. However,
more detailed information on the facies architecture of these
proximal foreland basin strata has revealed their complex
nature (Süss 1996). Prograding and laterally switching
subdeltas in combination with subsidence and sediment compaction created interdistributary swamps and shallow
embayments favorable for the generation of various sorts of
peat. This autocyclic mechanism is responsible for the splitting and pinching out of coal seams and other facies types.
Overprint of the deltaic environment by high-frequency (100
to 400 ka) sea-level changes of large amplitude created
shallowing-upward parasequences, ranging from 60 to 100 m
in thickness. The lower part ofthese parasequences is characterized by marine flooding surfaces, the upper part by
paleosols indicating continued regression of the sea.
7.6.3 Coastal Lagoons and Lowland LakeslPlayas
Specific marine flooding events mayaiso affect large
lagoons or lake basins normally separated from the sea
(Fig. 7 .29b). Then either freshwater and brackish water
or high-salinity brines are intermittently replaced by
environments approaching or reaching normal marine
conditions and their corresponding biota. Individual
layers or thicker strata reflecting these conditions indicate the "maximum flooding surface" of such events.
Relatively short marine ingress ions into long-persisting continental basins, in particular intracratonic
basins, are known from many countries and epochs.
The ingressions may leave behind marine shales and
carbonate horizons or, in some cases, thick gypsum
layers which can hardly be explained by the dissolved
load of entering rivers poor in sulfate. Flooding of coal
swamps and the deposition of marine shales have been
mentioned above. The influence of the sea-Ievel
highstand may be overprinted by climate change leading to greatly variable depositional systems.
The Germanic facies province of the Triassic in Europe
mainly consists of continental red beds altemating with some
marine beds caused by a few prominent and several minor
marine flooding periods (Sect. 12.3.2).
These sequences have been recently interpreted in terms
ofsequence stratigraphy (e.g. Aigner and Bachmann 1992).
At the transition from the Middle Triassic (Muschelkalk) to
the Upper Triassic (Keuper) minor marine flooding events
have affected not only coastal plain deposits (dolomite overlain by shale with coal) but also generated coastal sabkhas
precipitating subtidal to supratidal gypsum (Aigner 1998) .
•
boniferous in North America (after Heckel 1994,
simplified). e Lower Miocene paleo-valley incised
along distal margin of northem Alpine fore land basin
into marine and freshwater molasse deposits, southem Germany (after Buchner et al. 1996, modified).
For further explanation see text