Alluvial Fans and Other Fluvial Distributary Systems
stitute sandstone sheets of the element LS (Figs. 8.8P)
8.66). Cotter and Graham (1991), in their studies of
ephemeralMfluvial sandstones of Devonian age in
Ireland, described examples of lithofacies Sl which
they likened to hummocky cross-stratification, and
concluded that the facies represented deposition
from transitional to upper-plane bedforms, similar
to those described by Cowan (1991). Channels are
poorly defined or absent altogether. Flood cycles
may be recognizable, showing cut-and-fill relief
along their basal bounding surfaces. Overbanlc fines
are rare to absent. Antidunes are common in the
modern stream deposits described by Langford and
Bracken (1987), but do not appear to have a very high
preservation potentiaL Evaporite crusts may occur
on the upper surface of sand beds, and disruption of
bedding by evaporite crystallization is common
(Mertz and Hubert 1990).
The succession of events in a single flood cycle
was described by Bromley (1991a), based on his
studies of the Jurassic Kayenta Formation of western
Colorado. He mapped a broad channel with gently
dipping margins that appears to have fo cused flow in
the initial stages of the flood (Fig. 8.67). At the beginning of the flood event, a phase of channel-floor
erosion and dune sedimentation occurred. During
peak discharge, flow extended in a shallow sheet ove r
a broad area, beyond the confines of the original,
poorly defined channel, and sheets of Sh were deposited. During the waning flood stage, incision of
these sheets occurred, and pockets of two- and threedimensional dunes (lithofacies Sp, St) were deposited.
8.3 Alluvial Fans
and Other Flwial Distributary Systems
Alluvial fans are a form of fluvial depositional system distinguished on the basis of geomorphic character rather than by a characteristic fluvial style. As
discussed here, the term is in broad, general use
amongst sedimentologists, but there is considerable
confusion and disagreement as to the range of types
of fluvial depositional systems that should be included under the heading of alluvial fans.
An alluvial fan is defined in the Glossary of Geology (Bates and jackson 1987, p. 17) as:
"A low, outspread, relatively flat to gently sloping mass of
loose rock material, shaped like an open fan or a segment
of a cone, deposited by a stream (esp. in a semiarid region)
at the place where it issues from a narrow mountain valley
245
upon a plain or broad valley, or where a tributary stream is
near or at its junction vvith the main stream, or wherever a
constriction in a valley abruptly ceases or the gradient of
the stream suddenly decreases; it is steepest near the
mouth of the valley where its apex points upstream, and it
slopes gently and convexly outward with gradually
decresing gradient."
Miall (1990, p. 345) suggested that:
"the term alluvial fan may be used for any entirely
nonmarine and nonlacustrine fluvial system whose channel network is distributary rather than con tributary. Fans
that prograde directly into a standing body of water are
termed fan deltas or coastal alluvial fans . ... Alluvial fans
and fan deltas are a sedimentary response to flow expansion at a basin margin."
The term "fan" places natural emphasis on the
surface geomorphology of this depositional system.
The radiating distributaries and the cone-shaped
architecture of the deposit are the two features that
would be the key elements of most geologists' definition of the term alluvial fan. However, there is considerable controversy amongst sedimentologists as
to the scope of the definition beyond this initial
point. Miall's (1990) definition (which reflects widespread usage) implies that the term has no unique
facies sense. As noted by Miall (1992, p. 134), there
are small, steep, gravel-dominated fans, within
many of which debris-slow processes are important;
there are sandy systems deposited by braided
streams; there are giant modern fans, such as the
Kosi of India, which grades from boulder conglomerate near the mountains to fine sand-silt-mud 140
km downslope at itS distal end; and there are finegrained, arid systems, termed "terminal fans" deposited by ephemeral flows that percolate and
dissipate into the substrate at their distal fringes.
Geomorphologically, these are all alluvial fans, and
all fall under the defmitions of this term provided by
Bates and jackson (1987) and Miall (1990, 1992). A
recent review by Stanistreet and McCarthy (1993)
has followed this inclusive approach. They described
a new class of large, low-energy fan based on the
example of the modern Okavango fan of Botswana,
and erected a simple classification of alluvial fans,
including all the types mentioned above. This is
described below. They also discussed comparisons
with submarine fans, a class of depositional system
a bout which there have also been problems of definition.
Contrasting with this approach is that of
McPherson and Blair (1993) and Blair and
McPherson (1994). These authors accepted many of
the standard components of the geomorphic definiM
tion of fans, noted above, including their conical,
stitute sandstone sheets of the element LS (Figs. 8.8P)
8.66). Cotter and Graham (1991), in their studies of
ephemeralMfluvial sandstones of Devonian age in
Ireland, described examples of lithofacies Sl which
they likened to hummocky cross-stratification, and
concluded that the facies represented deposition
from transitional to upper-plane bedforms, similar
to those described by Cowan (1991). Channels are
poorly defined or absent altogether. Flood cycles
may be recognizable, showing cut-and-fill relief
along their basal bounding surfaces. Overbanlc fines
are rare to absent. Antidunes are common in the
modern stream deposits described by Langford and
Bracken (1987), but do not appear to have a very high
preservation potentiaL Evaporite crusts may occur
on the upper surface of sand beds, and disruption of
bedding by evaporite crystallization is common
(Mertz and Hubert 1990).
The succession of events in a single flood cycle
was described by Bromley (1991a), based on his
studies of the Jurassic Kayenta Formation of western
Colorado. He mapped a broad channel with gently
dipping margins that appears to have fo cused flow in
the initial stages of the flood (Fig. 8.67). At the beginning of the flood event, a phase of channel-floor
erosion and dune sedimentation occurred. During
peak discharge, flow extended in a shallow sheet ove r
a broad area, beyond the confines of the original,
poorly defined channel, and sheets of Sh were deposited. During the waning flood stage, incision of
these sheets occurred, and pockets of two- and threedimensional dunes (lithofacies Sp, St) were deposited.
8.3 Alluvial Fans
and Other Flwial Distributary Systems
Alluvial fans are a form of fluvial depositional system distinguished on the basis of geomorphic character rather than by a characteristic fluvial style. As
discussed here, the term is in broad, general use
amongst sedimentologists, but there is considerable
confusion and disagreement as to the range of types
of fluvial depositional systems that should be included under the heading of alluvial fans.
An alluvial fan is defined in the Glossary of Geology (Bates and jackson 1987, p. 17) as:
"A low, outspread, relatively flat to gently sloping mass of
loose rock material, shaped like an open fan or a segment
of a cone, deposited by a stream (esp. in a semiarid region)
at the place where it issues from a narrow mountain valley
245
upon a plain or broad valley, or where a tributary stream is
near or at its junction vvith the main stream, or wherever a
constriction in a valley abruptly ceases or the gradient of
the stream suddenly decreases; it is steepest near the
mouth of the valley where its apex points upstream, and it
slopes gently and convexly outward with gradually
decresing gradient."
Miall (1990, p. 345) suggested that:
"the term alluvial fan may be used for any entirely
nonmarine and nonlacustrine fluvial system whose channel network is distributary rather than con tributary. Fans
that prograde directly into a standing body of water are
termed fan deltas or coastal alluvial fans . ... Alluvial fans
and fan deltas are a sedimentary response to flow expansion at a basin margin."
The term "fan" places natural emphasis on the
surface geomorphology of this depositional system.
The radiating distributaries and the cone-shaped
architecture of the deposit are the two features that
would be the key elements of most geologists' definition of the term alluvial fan. However, there is considerable controversy amongst sedimentologists as
to the scope of the definition beyond this initial
point. Miall's (1990) definition (which reflects widespread usage) implies that the term has no unique
facies sense. As noted by Miall (1992, p. 134), there
are small, steep, gravel-dominated fans, within
many of which debris-slow processes are important;
there are sandy systems deposited by braided
streams; there are giant modern fans, such as the
Kosi of India, which grades from boulder conglomerate near the mountains to fine sand-silt-mud 140
km downslope at itS distal end; and there are finegrained, arid systems, termed "terminal fans" deposited by ephemeral flows that percolate and
dissipate into the substrate at their distal fringes.
Geomorphologically, these are all alluvial fans, and
all fall under the defmitions of this term provided by
Bates and jackson (1987) and Miall (1990, 1992). A
recent review by Stanistreet and McCarthy (1993)
has followed this inclusive approach. They described
a new class of large, low-energy fan based on the
example of the modern Okavango fan of Botswana,
and erected a simple classification of alluvial fans,
including all the types mentioned above. This is
described below. They also discussed comparisons
with submarine fans, a class of depositional system
a bout which there have also been problems of definition.
Contrasting with this approach is that of
McPherson and Blair (1993) and Blair and
McPherson (1994). These authors accepted many of
the standard components of the geomorphic definiM
tion of fans, noted above, including their conical,