Chapter 10
Fhnv!:rul l:leposit!ona! Systems amll Alltogenic Sedimentary Controls
10.1 Introduction
Beerbower ( 1964) was the first to clearly distinguish
between within-basin and extrabasinal sedimentary
controls. The first, which he termed autocyclicity,
refers to the redistribution of sediment within a
depositional system as a result of processes inherent
to the system, such as channel migration and bar
development. As discussed in Sect. 2.3.6.2, during
the 1950s and 1960s it was realized that fi ning-upward cycles in fluvial systems could be explained by
such processes. The term autogenic processes is now
preferred, because they are not necessarily strictly
cyclic in nature. Much of the sedimentological research in the 1960s and 1970s was concerned with the
recognition and description of autogenic processes,
as discussed in Chap. 2. The purpose of the present
chapter is to review the present state of our understanding of the subject. Extrabasinal, or allogenic
(formerly allocyclic), processes, are discussed in
Chap. 11.
The most important autogenic processes are
those which lead to changes in the position of channels. Channels may change in position and shape by
gradual lateral movement, or they may change position entirely to another part of the floodplain by the
process termed avulsion.
10.2 The Evolution
of Distributary Fluvial Systems
Distributary systems include alluvial fans and deltas.
They are characterized by networks of channels radiating from a single trunk channel. There may be one
or more active channels, together with several or
many dry channels that are occupied only during
floods or as a result of the autogenic processes described in this section. The current controversy regarding the definition of the term "alluvial fan" and
the range of distributary fluvial systems that should
be included under this heading is discussed in Sect.
8.3. This section is concerned primarily with what
Blair and McPherson (1994) term braid deltas.
There is a very large literature on alluvial fans and
other distributary systems, dealing with such data as
depositional processes, downstream changes in
slope, sediment grain size, sorting, fades, facies associations, and clast shape (Denny 1967; Bull 1972,
1977; Heward 1978a; Nilsen 1985; Rachocki and
Church 1990; Fraser and DeCelles 1992; Blair and
McPherson 1994). A significant body of field data is
presented in these publications, together with much
useful theoretical discussion. A summary of this
would require a book of its own, and the discussion
below is limited to a presentation of the more important concepts which control large-scale fan architecture and vertical profile. The main theoretical
framework for interpretation has been built from
studies of modern and ancient gravelly braided systems, supplemented by experimental work on small
laboratory models. However, it can be shown that
the same principles apply to all distributary systems,
such as terminal "fans" and the giant rivers of the
Himalayan foredeep.
Basin-margin gravel rivers are highly susceptible
to allogenic influences, especially tectonism. Much
research has been published on the sequences and
megasequences in the resulting deposits generated
by pulses of basin-margin faulti ng (e.g., Steel et a!.
1977; Steel and Aasheim 1978; Heward 1978a; Blair
and Bilodeau 1988). These processes act over a physical scale and time scale corresponding to sediment
groups 8 and above of this book (Table 9.1) and are
discussed in Chap. 11. However, autogenic processes
are of equal importance. They concern primarily
depositional processes and products of group 7 and
are the subject of this section.
There are two main autogenic processes in fluvial
distributary systems, the avulsive shifting of distributary channels and a cycle of trenching and
backfilling. The experimental work of Schumm et a!.
(1987), and the observations of DeCelles et a!. (1991)
and Fraser and DeCelles (1992) suggest that these
Fhnv!:rul l:leposit!ona! Systems amll Alltogenic Sedimentary Controls
10.1 Introduction
Beerbower ( 1964) was the first to clearly distinguish
between within-basin and extrabasinal sedimentary
controls. The first, which he termed autocyclicity,
refers to the redistribution of sediment within a
depositional system as a result of processes inherent
to the system, such as channel migration and bar
development. As discussed in Sect. 2.3.6.2, during
the 1950s and 1960s it was realized that fi ning-upward cycles in fluvial systems could be explained by
such processes. The term autogenic processes is now
preferred, because they are not necessarily strictly
cyclic in nature. Much of the sedimentological research in the 1960s and 1970s was concerned with the
recognition and description of autogenic processes,
as discussed in Chap. 2. The purpose of the present
chapter is to review the present state of our understanding of the subject. Extrabasinal, or allogenic
(formerly allocyclic), processes, are discussed in
Chap. 11.
The most important autogenic processes are
those which lead to changes in the position of channels. Channels may change in position and shape by
gradual lateral movement, or they may change position entirely to another part of the floodplain by the
process termed avulsion.
10.2 The Evolution
of Distributary Fluvial Systems
Distributary systems include alluvial fans and deltas.
They are characterized by networks of channels radiating from a single trunk channel. There may be one
or more active channels, together with several or
many dry channels that are occupied only during
floods or as a result of the autogenic processes described in this section. The current controversy regarding the definition of the term "alluvial fan" and
the range of distributary fluvial systems that should
be included under this heading is discussed in Sect.
8.3. This section is concerned primarily with what
Blair and McPherson (1994) term braid deltas.
There is a very large literature on alluvial fans and
other distributary systems, dealing with such data as
depositional processes, downstream changes in
slope, sediment grain size, sorting, fades, facies associations, and clast shape (Denny 1967; Bull 1972,
1977; Heward 1978a; Nilsen 1985; Rachocki and
Church 1990; Fraser and DeCelles 1992; Blair and
McPherson 1994). A significant body of field data is
presented in these publications, together with much
useful theoretical discussion. A summary of this
would require a book of its own, and the discussion
below is limited to a presentation of the more important concepts which control large-scale fan architecture and vertical profile. The main theoretical
framework for interpretation has been built from
studies of modern and ancient gravelly braided systems, supplemented by experimental work on small
laboratory models. However, it can be shown that
the same principles apply to all distributary systems,
such as terminal "fans" and the giant rivers of the
Himalayan foredeep.
Basin-margin gravel rivers are highly susceptible
to allogenic influences, especially tectonism. Much
research has been published on the sequences and
megasequences in the resulting deposits generated
by pulses of basin-margin faulti ng (e.g., Steel et a!.
1977; Steel and Aasheim 1978; Heward 1978a; Blair
and Bilodeau 1988). These processes act over a physical scale and time scale corresponding to sediment
groups 8 and above of this book (Table 9.1) and are
discussed in Chap. 11. However, autogenic processes
are of equal importance. They concern primarily
depositional processes and products of group 7 and
are the subject of this section.
There are two main autogenic processes in fluvial
distributary systems, the avulsive shifting of distributary channels and a cycle of trenching and
backfilling. The experimental work of Schumm et a!.
(1987), and the observations of DeCelles et a!. (1991)
and Fraser and DeCelles (1992) suggest that these