246
radiating form, their steep depositional slopes, and
their association with high-relief source areas. They
pointed out a significant contrast between fans and
most other fluvial systems. The latter typically exhibit concave-up transverse profiles, the channel being the lowest point, whereas fan systems are
characterized by convex-up transverse profiles,
which lead to overbank flow and sheet flooding of
bed load. However, these authors argue that there is
a distinct break in the range of longitudinal slopes
exhibited by the various types of fluvial distributary
system. They recommended limiting the term alluvial fan to steep systems, which have a range of slopes
between 1.5' and 2s• (slope: 0.026-0.466). These fans
are characterized by sediment-gravity-flow deposits,
especially debris flows, by hyperconcentrated flows
and sheetfloods. Traction-current deposits are rare
to absent. According to the same authors, fluvial
depositional systems having slopes in the range
0.4-1.5' (0.007-0.026) are rare, with most meandering and braided fluvial systems, including the radial,
distributary systems that many authors would classify as fans, exhibiting slopes less than 0.4°. This
provides a natural break, which they proposed to
emphasize in their classification. They recommended excluding depositional systems on slopes of
< IS ( < 0.026) from the category of alluvial fans,
mainly on the basis of an historical analysis of the
evolution of the term fan. They pointed out that the
original work of such researchers as Drew ( 1873) and
Surell (1841, 1870), which helped to establish the
term ((fan'', dealt with small, steep, coarse-grained
fans (Sect. 2.2.2). Heward's ( 1978a) important review
of fan depositional cycles also fo cuses on coarsegrained fans, although he did not discriminate or
subdivide fans on the basis of sediment transport
mechanisms. Blair and McPherson (1994) suggested
that the inclusion of fa n-shaped glacial outwash dispersal systems under the heading of the term alluvial
fan serves no useful purpose.
Following this approach, fine-grained, fanshaped deposits, such as those of the Kosi River,
terminal fans, and those not associated with mountainous uplifts, would not be included under the
heading of alluvial fans. They would be termed
distributary fluvial systems or braid deltas. While
this proposal is clear, and may be applied with some
consistency, at least to modern environments where
slopes can be measured, it leaves many depositional
systems and deposits that have traditionally been
termed fans out of the classification. This may be of
little importance in the case of sandy systems, such
as the Kosi fa n, even though the pattern of channel
wandering and avulsion documented on this fan
Fluvial Styles and Facies Models
(Holmes 1965; Gole and Chitale 1966; Wells and Dorr
1987; Singh et al. 1993) has been cited as a "textbook"
example of one style of channel evolution within fans
(e.g., Collinson 1986). Also, the term "terminal fan"
(Mukerji 1976; Friend 1978; Parkash et a!. 1983) is
now widely used (e.g., Kelly and Olsen 1993), but
most terminal fans are not alluvial fans according to
the definition of Blair and McPherson (1994).
Continuing detailed work on modern and Recent
fans suggests that the proposals of Blair and
McPherson (1994) may need modification. Nemec
and Postma (1993) provided a detailed description
of some Quaternary alluvial fans in Crete that do not
fit their definitions. A suite of fan deposits there has
preserved depositional dips of up to 13', yet the
deposits consist largely of clast-supported streamflow gravels forming gravel sheets (element GB).
Use of the restricted definition of Blair and
McPherson (1994) will lead to the recognition offans
in the ancient record on the basis of their facies
characteristics alone, the deposits being coarsegrained debris-flow and related facies banked
against an uplifted source area. Such classification
already is implied by the use of such terms as
((fanglomerate" for ancient conglomerates, even
where evidence of the geomorphology of the original
depositional system is not available. Such a designation will likely be suggested without evidence of the
distributary nature of the fluvial system, a feature
that is difficult to prove in the rock record without
detailed paleocurrent evidence. Whether the use of
the Blair and McPherson (1994) definition in this
way will prove to be useful and will serve to reduce
controversy and confusion remains to be learned
from experience. In the meantime, the more inclusive classification of Stanistreet and McCarthy
(1993) is provided below.
In this classification (Fig. 8.68), it is recognized
that various fluvial processes may occur in different
proportions, depending on climate, the nature of
the source terrane, etc. This led Stanistreet and
McCarthy (1993) to propose a triangular classification that illustrates the varying importance of the
three main processes, sediment-gravity flows, braiding, and meandering (Fig. 8.69). The first class offan
in their classification is the debris-flow-dominated
fan, the ouly type accepted by Blair and McPherson
(1994) as a ('true" fan. These fans are small, typically
less than 10 km in radiallength (Fig. 8.70), and steep
(slope > 0.1). They are particularly common in arid
regions, where sediment-gravity flow processes are
· more common (but are not exclusively found in this
climatic setting, as detailed in Sect. 12.5). Blair and
McPherson (1994) recognized two varieties. The first
radiating form, their steep depositional slopes, and
their association with high-relief source areas. They
pointed out a significant contrast between fans and
most other fluvial systems. The latter typically exhibit concave-up transverse profiles, the channel being the lowest point, whereas fan systems are
characterized by convex-up transverse profiles,
which lead to overbank flow and sheet flooding of
bed load. However, these authors argue that there is
a distinct break in the range of longitudinal slopes
exhibited by the various types of fluvial distributary
system. They recommended limiting the term alluvial fan to steep systems, which have a range of slopes
between 1.5' and 2s• (slope: 0.026-0.466). These fans
are characterized by sediment-gravity-flow deposits,
especially debris flows, by hyperconcentrated flows
and sheetfloods. Traction-current deposits are rare
to absent. According to the same authors, fluvial
depositional systems having slopes in the range
0.4-1.5' (0.007-0.026) are rare, with most meandering and braided fluvial systems, including the radial,
distributary systems that many authors would classify as fans, exhibiting slopes less than 0.4°. This
provides a natural break, which they proposed to
emphasize in their classification. They recommended excluding depositional systems on slopes of
< IS ( < 0.026) from the category of alluvial fans,
mainly on the basis of an historical analysis of the
evolution of the term fan. They pointed out that the
original work of such researchers as Drew ( 1873) and
Surell (1841, 1870), which helped to establish the
term ((fan'', dealt with small, steep, coarse-grained
fans (Sect. 2.2.2). Heward's ( 1978a) important review
of fan depositional cycles also fo cuses on coarsegrained fans, although he did not discriminate or
subdivide fans on the basis of sediment transport
mechanisms. Blair and McPherson (1994) suggested
that the inclusion of fa n-shaped glacial outwash dispersal systems under the heading of the term alluvial
fan serves no useful purpose.
Following this approach, fine-grained, fanshaped deposits, such as those of the Kosi River,
terminal fans, and those not associated with mountainous uplifts, would not be included under the
heading of alluvial fans. They would be termed
distributary fluvial systems or braid deltas. While
this proposal is clear, and may be applied with some
consistency, at least to modern environments where
slopes can be measured, it leaves many depositional
systems and deposits that have traditionally been
termed fans out of the classification. This may be of
little importance in the case of sandy systems, such
as the Kosi fa n, even though the pattern of channel
wandering and avulsion documented on this fan
Fluvial Styles and Facies Models
(Holmes 1965; Gole and Chitale 1966; Wells and Dorr
1987; Singh et al. 1993) has been cited as a "textbook"
example of one style of channel evolution within fans
(e.g., Collinson 1986). Also, the term "terminal fan"
(Mukerji 1976; Friend 1978; Parkash et a!. 1983) is
now widely used (e.g., Kelly and Olsen 1993), but
most terminal fans are not alluvial fans according to
the definition of Blair and McPherson (1994).
Continuing detailed work on modern and Recent
fans suggests that the proposals of Blair and
McPherson (1994) may need modification. Nemec
and Postma (1993) provided a detailed description
of some Quaternary alluvial fans in Crete that do not
fit their definitions. A suite of fan deposits there has
preserved depositional dips of up to 13', yet the
deposits consist largely of clast-supported streamflow gravels forming gravel sheets (element GB).
Use of the restricted definition of Blair and
McPherson (1994) will lead to the recognition offans
in the ancient record on the basis of their facies
characteristics alone, the deposits being coarsegrained debris-flow and related facies banked
against an uplifted source area. Such classification
already is implied by the use of such terms as
((fanglomerate" for ancient conglomerates, even
where evidence of the geomorphology of the original
depositional system is not available. Such a designation will likely be suggested without evidence of the
distributary nature of the fluvial system, a feature
that is difficult to prove in the rock record without
detailed paleocurrent evidence. Whether the use of
the Blair and McPherson (1994) definition in this
way will prove to be useful and will serve to reduce
controversy and confusion remains to be learned
from experience. In the meantime, the more inclusive classification of Stanistreet and McCarthy
(1993) is provided below.
In this classification (Fig. 8.68), it is recognized
that various fluvial processes may occur in different
proportions, depending on climate, the nature of
the source terrane, etc. This led Stanistreet and
McCarthy (1993) to propose a triangular classification that illustrates the varying importance of the
three main processes, sediment-gravity flows, braiding, and meandering (Fig. 8.69). The first class offan
in their classification is the debris-flow-dominated
fan, the ouly type accepted by Blair and McPherson
(1994) as a ('true" fan. These fans are small, typically
less than 10 km in radiallength (Fig. 8.70), and steep
(slope > 0.1). They are particularly common in arid
regions, where sediment-gravity flow processes are
· more common (but are not exclusively found in this
climatic setting, as detailed in Sect. 12.5). Blair and
McPherson (1994) recognized two varieties. The first