322
Fluvial Depositional Systems and Autogenic Sedimentary Controls
10.3.2 Avulsion in Braided Fluvial Systems
Braided rivers tend to occupy most of their alluvial
valley, leaving little room for floodplains, and therefore only minor areas for the channel to avulse into.
However, avulsive shifting is a significant sedimentary control, and large- and small-scale effects on
stratigraphic architecture can result, as discussed in
this section.
By far the most well-known examples of the largescale avulsive shifting of braided rivers are those of
the Kosi River in India, and the Yellow River in
China. The Kosi River is referred to elsewhere in this
book as an alluvial fan (e.g., Sect. 8.3), and the avulsion processes there are described in Sect. 10.2. However, it may serve as the type example of a particular
avulsion mechanism, that which is triggered by
aggradational elevation of the channel on the floodplain surface. This process seems to be particularly
important in braided systems, which are characterized by large bedloads and by rapid aggradation of
channels and tars. Other types of distributive systems, such as terminal fans, also show this type of
autogenic avulsion process (Sect. 10.2). Richards et
a!. (1993) noted that other fans in the Indo-Gangetic
plains also show avulsive behavior similar to that of
the Kosi. Several 0ther examples of avulsive channel
and bar changes in modern rivers are discussed in
Best and Bristow (1993).
Where the river is in the form of a distributive
system, as in the case of «inland deltas" such as
the Kosi River or terminal fans in arid regions, the
channel and adjacent fl oodplains are elevated with
respect to the regional surface as a result of aggradation. Processes such as those which occur in the Kosi
River system are therefore likely to be characteristic.
West
The spectacular and catastrophic diversions of the
Yellow River in China are of this type (Holmes 1965;
Li and Finlayson 1993).
The Eocene Escanilla Formation of the Spanish
Pyrenees appears to represent an exception to the
generalization that in braided systems channels occupy most of the alluvial valley (Dreyer et a!. 1993;
Bentham et a!. 1993; see Sect. 8.2). Channel-fill architecture indicates a braided fluvial style. Fine-grained
deposits total more than 40% of the succession, by
volume. Bentham et a!. (1993) attributed this fluvial
style to rapid subsidence and frequent avulsion
within an unconfined basin. There are indications
that the depositional system was distributary in
nature, and there may therefore have been a tendency for the depositional su�face to have been convex-up, which would have facilitated avulsion, as in
the various alluvial fans described earlier in this
chapter.
The alluvial architecture of the Old Red Sandstone (Lower Devonian) as exposed in coastal sections in southwest Wales, is discussed and illustrated
in Sect. 9.5.1 (Figs. 9.25, 9.26) and serves as a good
example of tightly documented regional autogenic
control of a largely braided fluvial system. Although
base-level change may be one contributing factor in
the development of this alluvial suite, autogenic
switching of the main channel belts on average about
every 8000 years was also suggested by Allen and
Williams (1982) as a major sedimentary control. The
depositional environment is interpreted as that of a
coastal-plain tidally influenced fluvial and mudflat
complex, in which the channels were oflow sinuosity
and may have been braided. Earlier work on these
beds by Allen (1974b), based on a much less extensive data base of vertical profiles, had led to speculaEast
Campbell's "channel system"
Fig. 10.12. Cross section of the Morrison Formation,
northern New Mexico, redrawn from the work of Campbell
(1976). Units preft.xed with "J" are members of the
Morrison Formation: Jmr Recapture Member; Jmw Westwater Canyon Member; Jmb Brushy Basin Member; Kd
Dakota Formation. The diagram was constructed by reconnaissance correlation of channels within the Weswater
Canyon Member, and the top of the diagram does not
correspond to the top of the Dakota Formation. rectangle
at left shows the area re-examined in detail by Cowan
(1991)
Fluvial Depositional Systems and Autogenic Sedimentary Controls
10.3.2 Avulsion in Braided Fluvial Systems
Braided rivers tend to occupy most of their alluvial
valley, leaving little room for floodplains, and therefore only minor areas for the channel to avulse into.
However, avulsive shifting is a significant sedimentary control, and large- and small-scale effects on
stratigraphic architecture can result, as discussed in
this section.
By far the most well-known examples of the largescale avulsive shifting of braided rivers are those of
the Kosi River in India, and the Yellow River in
China. The Kosi River is referred to elsewhere in this
book as an alluvial fan (e.g., Sect. 8.3), and the avulsion processes there are described in Sect. 10.2. However, it may serve as the type example of a particular
avulsion mechanism, that which is triggered by
aggradational elevation of the channel on the floodplain surface. This process seems to be particularly
important in braided systems, which are characterized by large bedloads and by rapid aggradation of
channels and tars. Other types of distributive systems, such as terminal fans, also show this type of
autogenic avulsion process (Sect. 10.2). Richards et
a!. (1993) noted that other fans in the Indo-Gangetic
plains also show avulsive behavior similar to that of
the Kosi. Several 0ther examples of avulsive channel
and bar changes in modern rivers are discussed in
Best and Bristow (1993).
Where the river is in the form of a distributive
system, as in the case of «inland deltas" such as
the Kosi River or terminal fans in arid regions, the
channel and adjacent fl oodplains are elevated with
respect to the regional surface as a result of aggradation. Processes such as those which occur in the Kosi
River system are therefore likely to be characteristic.
West
The spectacular and catastrophic diversions of the
Yellow River in China are of this type (Holmes 1965;
Li and Finlayson 1993).
The Eocene Escanilla Formation of the Spanish
Pyrenees appears to represent an exception to the
generalization that in braided systems channels occupy most of the alluvial valley (Dreyer et a!. 1993;
Bentham et a!. 1993; see Sect. 8.2). Channel-fill architecture indicates a braided fluvial style. Fine-grained
deposits total more than 40% of the succession, by
volume. Bentham et a!. (1993) attributed this fluvial
style to rapid subsidence and frequent avulsion
within an unconfined basin. There are indications
that the depositional system was distributary in
nature, and there may therefore have been a tendency for the depositional su�face to have been convex-up, which would have facilitated avulsion, as in
the various alluvial fans described earlier in this
chapter.
The alluvial architecture of the Old Red Sandstone (Lower Devonian) as exposed in coastal sections in southwest Wales, is discussed and illustrated
in Sect. 9.5.1 (Figs. 9.25, 9.26) and serves as a good
example of tightly documented regional autogenic
control of a largely braided fluvial system. Although
base-level change may be one contributing factor in
the development of this alluvial suite, autogenic
switching of the main channel belts on average about
every 8000 years was also suggested by Allen and
Williams (1982) as a major sedimentary control. The
depositional environment is interpreted as that of a
coastal-plain tidally influenced fluvial and mudflat
complex, in which the channels were oflow sinuosity
and may have been braided. Earlier work on these
beds by Allen (1974b), based on a much less extensive data base of vertical profiles, had led to speculaEast
Campbell's "channel system"
Fig. 10.12. Cross section of the Morrison Formation,
northern New Mexico, redrawn from the work of Campbell
(1976). Units preft.xed with "J" are members of the
Morrison Formation: Jmr Recapture Member; Jmw Westwater Canyon Member; Jmb Brushy Basin Member; Kd
Dakota Formation. The diagram was constructed by reconnaissance correlation of channels within the Weswater
Canyon Member, and the top of the diagram does not
correspond to the top of the Dakota Formation. rectangle
at left shows the area re-examined in detail by Cowan
(1991)