350
Chapter 7 Sequences, Minor Cycles, and Event Stratigraphy
Repeated sequences of larger areal extent form on
alluvial plains fed by braided and meandering river
systems (Sect. 2.2.3). As a result of lateral channel
migration, relatively coarse-grained channel fills can
become amalgamated to wide, elongated sand sheets.
Particularly in braided systems, older channel fills may
be partially reworked and their material incorporated
into subsequent channel fills. Idealized channel sections show a fining-upward trend which is commonly
terminated by the erosional face of the subsequent
channel. Meandering systems show channel fills with
well developed fining-upward character and less erosional truncation when aggradation of flood deposits
predominates over channel sediments. Abandoned
channels (ox bow lakes) may be filled with mud or
organic detritus (cf. Fig. 2.16). Pedogenesis characterizes areas of slow or absent sediment aggradation during time intervals of at least some thousand years.
If a kind of equilibrium between subsidence and
sediment aggradation can be maintained (steady-state
conditions), the fluvial systems thus generate
autocyclic or autogenetic sedimentary cycles or sequences with fining-upward channel fills. The thickness of these sequences reflects the depth of the channels, and their architectural style remains more or less
constant with time.
Allogenetic Controls
Autogenetic fluvial cycles related solely to the dynamics of the depositional system itself may be superposed
by processes of a more regional or global nature. To
better understand these allogenetic processes, some
principal features of the complex fluvial systems are
summarized in Figure 7 .32a (cf. Fig. 2.20). The overall
gradient of river systems is controlled by processes in
both the upper reaches (sediment source) and lower
reaches where sediment aggradation takes place. As a
result, the base-level (stream-gradient) profiles ofriver
systems can vary significantly (Fig. 7.32d). Some authors have defined an "stream equilibrium profile"
which acts as a "pseudo-base level" controlling erosion
or deposition of the fluvial system. If the landscape
and the actual stream bed lie above this level, erosional
processes dominate. Where (theoretical) stream-profile
Fig. 7.33. a Example of a fluvial half-graben fill,
Tnassic, central Spain. Note upsection changes in
channel connectedness, grain sizes, and variations in
paleo-current directions. (After Sanchez-Moya et al.
1996, simplified). b Idealized cross-section of continental interior basin, Upper Triassic (Keuper), southem Germany, demonstrating prograding and
backstepping fluvial facies belts as controlled by
base-level change (noted in two different ways).
Isochrones do not follow facies boundaries. (After
Konstanty and Ricken 1997, modified). c Delayed
changes occur for some time above the land surface,
fluvial sediment may accumulate at increasing and decreasing rates generating cyclic sequences in conformable strata. The channel pattern, length and specific
gradient of individual channels, and their bedload try
to adjust to changing environmental conditions
(stream-profile, climate, amount of runoff, sediment
supply, etc.). However, there may be a considerable
lag time between an actual change in one or several of
the controlling variables and the reaction ofthe fluvial
system and its attempt to reestablish equilibrium conditions. A pseudo base-level fluctuation of a fluvial system far away from the sea may be completely out of
phase with relative sea-Ievel changes feit along the
coastline. Furthermore, a stream equilibrium profile is
not an ultimate base level. Rivers often change their
character and thus also their stream gradient with time.
These problems have been recently discussed by several authors (e.g. Cross and Homewood 1997; Dalrymple et al.
1998; Ethridge et al. 1998). A generally accepted concept
seems to be still missing. The number of publications about
continental sequence stratigraphy is steadily increasing (e.g.
Ramon and Cross 1997; Shanley and McCabe 1998).
In spite of these complications, the generalized rules
outlined in Figure 7.32a,d may help in gaining a better
understanding of fluvial sequence stratigraphy. The
response of fluvial systems to base-level change in the
downstream region is demonstrated in Figure 7.32b,c.
As long as the river can maintain a kind of equilibrium
profile (steady-state), fluvial sequences do not display
any significant vertical trend. However, a rise in base
level resulting either from decreased subsidence or
increased sediment accumulation, diminishes the river
gradient and forces backstepping (upstream retreat) of
the fluvial facies belts. For example, the braided channel system with relatively coarse-grained material
shifts headward. If this trend continues, the slope of
the fluvial basin considered may become very flat and,
depending on the weathering processes in the source
area, only allow anastomosing rivers to deposit finegrained sediments. Ifthe basin becomes hydrologically
closed, it is finally transformed into a lake or playa the
sediments ofwhich mark the top offining-upward sections.
response of fluvial system to constant subsidence of
continental interior basin (as b) may cause alternating prograding and backstepping of fluvial facies
belts (self-regulating system). d Types of continental
sequences bounded either by erosional unconformities, soils and duricrusts (SB 1) or by strata deposited
at the transition from base-level rise to fall (corresponding to maximum flooding surface). (Based on
several sources, e.g. Olsen 1995; Tandon and Gibling
1997)
Chapter 7 Sequences, Minor Cycles, and Event Stratigraphy
Repeated sequences of larger areal extent form on
alluvial plains fed by braided and meandering river
systems (Sect. 2.2.3). As a result of lateral channel
migration, relatively coarse-grained channel fills can
become amalgamated to wide, elongated sand sheets.
Particularly in braided systems, older channel fills may
be partially reworked and their material incorporated
into subsequent channel fills. Idealized channel sections show a fining-upward trend which is commonly
terminated by the erosional face of the subsequent
channel. Meandering systems show channel fills with
well developed fining-upward character and less erosional truncation when aggradation of flood deposits
predominates over channel sediments. Abandoned
channels (ox bow lakes) may be filled with mud or
organic detritus (cf. Fig. 2.16). Pedogenesis characterizes areas of slow or absent sediment aggradation during time intervals of at least some thousand years.
If a kind of equilibrium between subsidence and
sediment aggradation can be maintained (steady-state
conditions), the fluvial systems thus generate
autocyclic or autogenetic sedimentary cycles or sequences with fining-upward channel fills. The thickness of these sequences reflects the depth of the channels, and their architectural style remains more or less
constant with time.
Allogenetic Controls
Autogenetic fluvial cycles related solely to the dynamics of the depositional system itself may be superposed
by processes of a more regional or global nature. To
better understand these allogenetic processes, some
principal features of the complex fluvial systems are
summarized in Figure 7 .32a (cf. Fig. 2.20). The overall
gradient of river systems is controlled by processes in
both the upper reaches (sediment source) and lower
reaches where sediment aggradation takes place. As a
result, the base-level (stream-gradient) profiles ofriver
systems can vary significantly (Fig. 7.32d). Some authors have defined an "stream equilibrium profile"
which acts as a "pseudo-base level" controlling erosion
or deposition of the fluvial system. If the landscape
and the actual stream bed lie above this level, erosional
processes dominate. Where (theoretical) stream-profile
Fig. 7.33. a Example of a fluvial half-graben fill,
Tnassic, central Spain. Note upsection changes in
channel connectedness, grain sizes, and variations in
paleo-current directions. (After Sanchez-Moya et al.
1996, simplified). b Idealized cross-section of continental interior basin, Upper Triassic (Keuper), southem Germany, demonstrating prograding and
backstepping fluvial facies belts as controlled by
base-level change (noted in two different ways).
Isochrones do not follow facies boundaries. (After
Konstanty and Ricken 1997, modified). c Delayed
changes occur for some time above the land surface,
fluvial sediment may accumulate at increasing and decreasing rates generating cyclic sequences in conformable strata. The channel pattern, length and specific
gradient of individual channels, and their bedload try
to adjust to changing environmental conditions
(stream-profile, climate, amount of runoff, sediment
supply, etc.). However, there may be a considerable
lag time between an actual change in one or several of
the controlling variables and the reaction ofthe fluvial
system and its attempt to reestablish equilibrium conditions. A pseudo base-level fluctuation of a fluvial system far away from the sea may be completely out of
phase with relative sea-Ievel changes feit along the
coastline. Furthermore, a stream equilibrium profile is
not an ultimate base level. Rivers often change their
character and thus also their stream gradient with time.
These problems have been recently discussed by several authors (e.g. Cross and Homewood 1997; Dalrymple et al.
1998; Ethridge et al. 1998). A generally accepted concept
seems to be still missing. The number of publications about
continental sequence stratigraphy is steadily increasing (e.g.
Ramon and Cross 1997; Shanley and McCabe 1998).
In spite of these complications, the generalized rules
outlined in Figure 7.32a,d may help in gaining a better
understanding of fluvial sequence stratigraphy. The
response of fluvial systems to base-level change in the
downstream region is demonstrated in Figure 7.32b,c.
As long as the river can maintain a kind of equilibrium
profile (steady-state), fluvial sequences do not display
any significant vertical trend. However, a rise in base
level resulting either from decreased subsidence or
increased sediment accumulation, diminishes the river
gradient and forces backstepping (upstream retreat) of
the fluvial facies belts. For example, the braided channel system with relatively coarse-grained material
shifts headward. If this trend continues, the slope of
the fluvial basin considered may become very flat and,
depending on the weathering processes in the source
area, only allow anastomosing rivers to deposit finegrained sediments. Ifthe basin becomes hydrologically
closed, it is finally transformed into a lake or playa the
sediments ofwhich mark the top offining-upward sections.
response of fluvial system to constant subsidence of
continental interior basin (as b) may cause alternating prograding and backstepping of fluvial facies
belts (self-regulating system). d Types of continental
sequences bounded either by erosional unconformities, soils and duricrusts (SB 1) or by strata deposited
at the transition from base-level rise to fall (corresponding to maximum flooding surface). (Based on
several sources, e.g. Olsen 1995; Tandon and Gibling
1997)