Growth of Present-Day Concepts, up to 1977
American Great Plains. All these papers have now
been extensively quoted in studies of ancient
braided stream deposits.
By contrast with meandering-river deposits,
those of braided rivers have generally been thought
of as somewhat random in depositional character,
and lacking recognizable cyclicity. Application of
the statistical technique of Markov chain analysis to
bedding sequences showed that this is not always the
case (Miall 1973; Cant and Walker 1976) and, in a
review of the braided river environment, Miall
(1977) developed four facies models for braided rivers, one of which was strongly cyclic. Generation of
these models was aided by the development of a
simple lithofacies coding system, which permitted
unified description of braided river deposits, including those already described in the literature. This
method has been widely adopted, and forms the
basis for lithofacies descriptions in this book (Chap.
5).
Cyclic braided deposits were assigned by Miall
(1977) to his Donjek model, but subsequent work
showed that this name would be best confined to
gravel-dominated cyclic deposits, as in the original
description of the Donjek sediments (Williams and
Rust 1969; Rust 1972). Cant and Walker (1976) and
Cant (1978) described a sand-dominated cycle,
which has been erected as a fifth model, and termed
the South Saskatchewan type (Miall 1978c). Rust
(1978b) discussed a slightly different grouping of
braided facies associations using his own field examples. Modern work on braided rivers and their
deposits is focusing on the details of the internal
architecture of their deposits (Sects. 2.4.3, 2.4.4.2,
Chap. 8).
2.3.6.4 Alluvial Fans
Because of their distinctive morphology, alluvial
fans have always been recognizable as discrete components of any alluvial system, and sedimentologists
have attempted to carry this distinction into their
interpretations of the ancient record. Several authors have distinguished an alluvial-fan facies from a
braided-river facies, which is misleading, because
much of the sedimentation that takes place on alluvial fans is brought about by the activity of braided
streams. The most distinctive components of alluvial
fan environments are debris-flow deposits, although
31
not all the depositional systems termed fans necessarily contain this facies (Miall 1978c; Rust 1978b).
Modern alluvial fans were studied by Tolman
(1909), who introduced the term "bajada" for the
laterally amalgamated fans that occur at the foot of
mountain slopes in many basins. The work of
Trowbridge (1911) has been referred to earlier.
Lawson (1913) coined the word "fanglomerate\ and
Blackwelder (1928) first appreciated the importance
of mudflows in alluvial-fan deposits. More recent,
detailed work on the geology of alluvial-fan deposits
by Blissenbach (1954) has been much quoted. Other
descriptions were given by Bull (1972) and
Boothroyd and Ashley (1975). Ancient fan deposits
were described by, amongst others, Nilsen (1968)
and Miall (1970a), both of whom used textural parameters and clast composition to map fan outlines
and transport directions. Heward (1978a) provided a
review of depositional processes on alluvial fans and
described some Carboniferous examples in Spain
(Heward 1978b). The first of these papers contains
an authoritative review of the autogenic and
allogenic mechanisms of cycle development in alluvial fans, and has become the standard work on the
subject. Boothroyd and Nummedal (1978) documented the depositional fades occurring on humid
fans.
Difficulties with the generalized concept of an
alluvial-fan depositional model were already apparent by the late 1970s. The work quoted in the
preceding paragraphs refers exclusively to graveldominated fans, but Friend (1978) described some
ancient · cone-shaped sandstone units that he compared to the deposits of modern "terminal" fans in
arid regions of lndia (Mukerji 1976). The giant fan of
the Kosi River was described by Gole and Chitale
( 1966), and its deposits have been discussed by Singh
et a!. (1993). This depositional system, so much
greater than the small fans normally conjured up by
the term alluvial fan, grades from boulder-dominated in proximal regions to a silt and fine sand
system at the toe, 60 km downslope. Later workers
became uneasy with the inclusion of fluvioglacial
outwash plains in the alluvial-fan category, even
though many of them are distributary systems. Important «fan'' papers, such as those by Boothroyd
and Ashley (1975) and Boothroyd and Nummedal
(1978) would be excluded, according to this approach. The current controversy in this area is discussed in Sect. 8.3.
American Great Plains. All these papers have now
been extensively quoted in studies of ancient
braided stream deposits.
By contrast with meandering-river deposits,
those of braided rivers have generally been thought
of as somewhat random in depositional character,
and lacking recognizable cyclicity. Application of
the statistical technique of Markov chain analysis to
bedding sequences showed that this is not always the
case (Miall 1973; Cant and Walker 1976) and, in a
review of the braided river environment, Miall
(1977) developed four facies models for braided rivers, one of which was strongly cyclic. Generation of
these models was aided by the development of a
simple lithofacies coding system, which permitted
unified description of braided river deposits, including those already described in the literature. This
method has been widely adopted, and forms the
basis for lithofacies descriptions in this book (Chap.
5).
Cyclic braided deposits were assigned by Miall
(1977) to his Donjek model, but subsequent work
showed that this name would be best confined to
gravel-dominated cyclic deposits, as in the original
description of the Donjek sediments (Williams and
Rust 1969; Rust 1972). Cant and Walker (1976) and
Cant (1978) described a sand-dominated cycle,
which has been erected as a fifth model, and termed
the South Saskatchewan type (Miall 1978c). Rust
(1978b) discussed a slightly different grouping of
braided facies associations using his own field examples. Modern work on braided rivers and their
deposits is focusing on the details of the internal
architecture of their deposits (Sects. 2.4.3, 2.4.4.2,
Chap. 8).
2.3.6.4 Alluvial Fans
Because of their distinctive morphology, alluvial
fans have always been recognizable as discrete components of any alluvial system, and sedimentologists
have attempted to carry this distinction into their
interpretations of the ancient record. Several authors have distinguished an alluvial-fan facies from a
braided-river facies, which is misleading, because
much of the sedimentation that takes place on alluvial fans is brought about by the activity of braided
streams. The most distinctive components of alluvial
fan environments are debris-flow deposits, although
31
not all the depositional systems termed fans necessarily contain this facies (Miall 1978c; Rust 1978b).
Modern alluvial fans were studied by Tolman
(1909), who introduced the term "bajada" for the
laterally amalgamated fans that occur at the foot of
mountain slopes in many basins. The work of
Trowbridge (1911) has been referred to earlier.
Lawson (1913) coined the word "fanglomerate\ and
Blackwelder (1928) first appreciated the importance
of mudflows in alluvial-fan deposits. More recent,
detailed work on the geology of alluvial-fan deposits
by Blissenbach (1954) has been much quoted. Other
descriptions were given by Bull (1972) and
Boothroyd and Ashley (1975). Ancient fan deposits
were described by, amongst others, Nilsen (1968)
and Miall (1970a), both of whom used textural parameters and clast composition to map fan outlines
and transport directions. Heward (1978a) provided a
review of depositional processes on alluvial fans and
described some Carboniferous examples in Spain
(Heward 1978b). The first of these papers contains
an authoritative review of the autogenic and
allogenic mechanisms of cycle development in alluvial fans, and has become the standard work on the
subject. Boothroyd and Nummedal (1978) documented the depositional fades occurring on humid
fans.
Difficulties with the generalized concept of an
alluvial-fan depositional model were already apparent by the late 1970s. The work quoted in the
preceding paragraphs refers exclusively to graveldominated fans, but Friend (1978) described some
ancient · cone-shaped sandstone units that he compared to the deposits of modern "terminal" fans in
arid regions of lndia (Mukerji 1976). The giant fan of
the Kosi River was described by Gole and Chitale
( 1966), and its deposits have been discussed by Singh
et a!. (1993). This depositional system, so much
greater than the small fans normally conjured up by
the term alluvial fan, grades from boulder-dominated in proximal regions to a silt and fine sand
system at the toe, 60 km downslope. Later workers
became uneasy with the inclusion of fluvioglacial
outwash plains in the alluvial-fan category, even
though many of them are distributary systems. Important «fan'' papers, such as those by Boothroyd
and Ashley (1975) and Boothroyd and Nummedal
(1978) would be excluded, according to this approach. The current controversy in this area is discussed in Sect. 8.3.