352
Chapter 7 Sequences, Minor Cycles, and Event Stratigraphy
In contrast, falling base level, caused by increased
subsidence and/or reduced sediment accumulation in
the lower fluvial reaches, leads to basinward
prograding of the relatively coarse-grained facies and
thus to coarsening-upward sequences. Falling base
level downstream of the location considered can also
cause valley incision into fluvial sediments deposited
earlier, i.e. during rising base level (Fig. 7.32b). After
turnover from falling to rising base level, fluvial basins
are commonly filled by a fining-upward sequence, often beginning with braided river sediments. The opposite trend, i.e. turnover from rising to falling base level,
produces a coarsening-upward sequence.
The cross section of a Triassic fluvial basin fill (Fig. 7.33a;
Bunter Sandstone, Spain; Sanchez-Moya et al. 1996) exempli fies an example oftwo tumovers in base level. The fluvial
architecture is characterized by the transition from a meandering to a braided system and back again to a meandering
system. At the same time, the mean grain sizes ofthe channel
fills, the inter-connectedness of the channeIs as weil as the
paleo-current patterns change.
Figure 7.33b shows an idealized example from southern
Germany (Keuper Formation, Triassie ), where the basin center was repeatedly occupied by a large playa lake recorded by
thin dolomitic limestone beds and some evaporites. Red
beds, some strata with calcrete, braided and meandering
rivers repeatedly prograded to and retreated from the basin
center. These features can be explained by base-level change,
but clirnatic change rnay also have played an important part
(see also Bourquin et al. 1998).
Prograding and backstepping of fluvial facies belts, as
demonstrated in the previous example, is not necessarily associated with periods of increased or decreased
subsidence. Even under conditions of constant subsidence, variations in fluvial sediment transport to the
basin center may produce fining and coarsening-upward sequences. During times of reduced sediment
supply and deposition, the basin center is lowering and
thus creating a steeper river gradient (Fig. 7 .33c). After
some lag time, the steepened gradient will in turn enhance the transport capacity of the river system and
thus cause prograding of the coarse fluvial facies. As
a result, the accumulation rate in the basin center will
again increase and fill the basin up to the initial level.
This process reduces further sediment supply, and the
same playas before can begin. Such aseries of processes may be referred to as a kind of "self-regulating"
mechanism which, of course, can be overprinted or
replaced by other processes. Climate change and/or
Fig. 7.34. a Eolian deposits with unconforrnities
("super surfaces") caused by episodes of deflation
and sabkha formation close to the groundwater table.
A relative water-table rise takes place in conjunction
with ongoing subsidence of the basin floor. Note that
the super surfaces represent more time than sediment
accumulation. (After Havholm and Kocurek 1994,
modified). b Deflation horizons (Stokes surfaces) in
tectonic movements in the source area can produce a
sirnilar pattern.
Sequence boundaries in continental systems can be
defined in different ways (Fig. 7.33d), as those in marine deposits. One possibility is to use unconforrnities
(SB 1) represented, e.g., by paleosols and/or the floor
of individual valleys or the erosional base of wide fluvial channel belts incised during falling base level. An
alternative is the tumover between rising and falling
base level, corresponding to the maximum flooding
surface of marine systems. This conforrnable and more
or less isochronous boundary is commonly weIl developed in the central parts of continental basins. Here,
the clay-rich facies of fluvial systems accumulates or,
as an alternative, lakes, playas, or coal swamps are established.
7.7.3 Eolian Sequences
The basis for recent attempts to apply modern concepts
of sequence stratigraphy to eolian sediments is the observation that these are often not solely of eolian origin. Eolian sands frequently alternate with fluvial and
playa deposits which can be defined as subunits of
sequences or may be used as sequence boundaries. In
addition, eolian sands often display specific erosional
unconforrnities (deflation surfaces, sometimes referred
to as super-surfaces) which can be traced over long
distances (Fig. 7.34). These "Stokes surfaces" (cf.
Sect. 2.3 and Fig. 2.21 d) are related to the groundwater
table within dune fields. Dry, non-cohesive sand above
the capillary fringe of thewater table can be blown
away, whereas moist sand resists wind erosion. This
phenomenon commonly leads to a discontinuous
buildup of eolian dune fields (Fig. 7.34a). Relatively
short periods of rapid sand accumulation are followed
by intervals of erosion (deflation), creating hiatuses in
the vertical sediment buildup. The groundwater table
serves as a sort of base level down to which deflation
can proceed.
In a subsiding basin the groundwater table generally
rises with time in relation to the initial basin floor. As
a result of climate change and other factors, the water
table may rise stepwise and sometimes also drop below
a previous level. In coastal sand dunes or in the vicinity of lakes, the groundwater table more or less follows
the sea or lake level change (Fig. 7.34b). Then the section bounded by two deflation surfaces reflects the
coastal sand dunes. c Stokes surfaces (continental
sequence boundaries) associated with inland sabkhas
and interdune fluvial deposits. (After Fryberger et al.
1988). d Large-scale facies models of the lower
Perrnian (Rotliegend, southem North Sea) displaying
the effects climate-induced base-level change from
arid to semi-arid. (After Yang and Nio 1993; Gaupp
et al. 1993, modified)
Chapter 7 Sequences, Minor Cycles, and Event Stratigraphy
In contrast, falling base level, caused by increased
subsidence and/or reduced sediment accumulation in
the lower fluvial reaches, leads to basinward
prograding of the relatively coarse-grained facies and
thus to coarsening-upward sequences. Falling base
level downstream of the location considered can also
cause valley incision into fluvial sediments deposited
earlier, i.e. during rising base level (Fig. 7.32b). After
turnover from falling to rising base level, fluvial basins
are commonly filled by a fining-upward sequence, often beginning with braided river sediments. The opposite trend, i.e. turnover from rising to falling base level,
produces a coarsening-upward sequence.
The cross section of a Triassic fluvial basin fill (Fig. 7.33a;
Bunter Sandstone, Spain; Sanchez-Moya et al. 1996) exempli fies an example oftwo tumovers in base level. The fluvial
architecture is characterized by the transition from a meandering to a braided system and back again to a meandering
system. At the same time, the mean grain sizes ofthe channel
fills, the inter-connectedness of the channeIs as weil as the
paleo-current patterns change.
Figure 7.33b shows an idealized example from southern
Germany (Keuper Formation, Triassie ), where the basin center was repeatedly occupied by a large playa lake recorded by
thin dolomitic limestone beds and some evaporites. Red
beds, some strata with calcrete, braided and meandering
rivers repeatedly prograded to and retreated from the basin
center. These features can be explained by base-level change,
but clirnatic change rnay also have played an important part
(see also Bourquin et al. 1998).
Prograding and backstepping of fluvial facies belts, as
demonstrated in the previous example, is not necessarily associated with periods of increased or decreased
subsidence. Even under conditions of constant subsidence, variations in fluvial sediment transport to the
basin center may produce fining and coarsening-upward sequences. During times of reduced sediment
supply and deposition, the basin center is lowering and
thus creating a steeper river gradient (Fig. 7 .33c). After
some lag time, the steepened gradient will in turn enhance the transport capacity of the river system and
thus cause prograding of the coarse fluvial facies. As
a result, the accumulation rate in the basin center will
again increase and fill the basin up to the initial level.
This process reduces further sediment supply, and the
same playas before can begin. Such aseries of processes may be referred to as a kind of "self-regulating"
mechanism which, of course, can be overprinted or
replaced by other processes. Climate change and/or
Fig. 7.34. a Eolian deposits with unconforrnities
("super surfaces") caused by episodes of deflation
and sabkha formation close to the groundwater table.
A relative water-table rise takes place in conjunction
with ongoing subsidence of the basin floor. Note that
the super surfaces represent more time than sediment
accumulation. (After Havholm and Kocurek 1994,
modified). b Deflation horizons (Stokes surfaces) in
tectonic movements in the source area can produce a
sirnilar pattern.
Sequence boundaries in continental systems can be
defined in different ways (Fig. 7.33d), as those in marine deposits. One possibility is to use unconforrnities
(SB 1) represented, e.g., by paleosols and/or the floor
of individual valleys or the erosional base of wide fluvial channel belts incised during falling base level. An
alternative is the tumover between rising and falling
base level, corresponding to the maximum flooding
surface of marine systems. This conforrnable and more
or less isochronous boundary is commonly weIl developed in the central parts of continental basins. Here,
the clay-rich facies of fluvial systems accumulates or,
as an alternative, lakes, playas, or coal swamps are established.
7.7.3 Eolian Sequences
The basis for recent attempts to apply modern concepts
of sequence stratigraphy to eolian sediments is the observation that these are often not solely of eolian origin. Eolian sands frequently alternate with fluvial and
playa deposits which can be defined as subunits of
sequences or may be used as sequence boundaries. In
addition, eolian sands often display specific erosional
unconforrnities (deflation surfaces, sometimes referred
to as super-surfaces) which can be traced over long
distances (Fig. 7.34). These "Stokes surfaces" (cf.
Sect. 2.3 and Fig. 2.21 d) are related to the groundwater
table within dune fields. Dry, non-cohesive sand above
the capillary fringe of thewater table can be blown
away, whereas moist sand resists wind erosion. This
phenomenon commonly leads to a discontinuous
buildup of eolian dune fields (Fig. 7.34a). Relatively
short periods of rapid sand accumulation are followed
by intervals of erosion (deflation), creating hiatuses in
the vertical sediment buildup. The groundwater table
serves as a sort of base level down to which deflation
can proceed.
In a subsiding basin the groundwater table generally
rises with time in relation to the initial basin floor. As
a result of climate change and other factors, the water
table may rise stepwise and sometimes also drop below
a previous level. In coastal sand dunes or in the vicinity of lakes, the groundwater table more or less follows
the sea or lake level change (Fig. 7.34b). Then the section bounded by two deflation surfaces reflects the
coastal sand dunes. c Stokes surfaces (continental
sequence boundaries) associated with inland sabkhas
and interdune fluvial deposits. (After Fryberger et al.
1988). d Large-scale facies models of the lower
Perrnian (Rotliegend, southem North Sea) displaying
the effects climate-induced base-level change from
arid to semi-arid. (After Yang and Nio 1993; Gaupp
et al. 1993, modified)