54
appropriate, being small rivers in Banff National
Park, in narrow mountain valleys undergoing degradation, quite different from the large channels
within the broad, aggrading alluvial plain of the
Alberta foreland basin (as revealed by sandstone
body geometries). Putnam (1993) has noted one of
the major difficulties in relating surface facies studies to subsui'face interpretations) and that is that
"sand" bodies interpreted from wireline logs may
include crevasse and levee deposits, which, combined with the width of the actual channel deposit,
would indicate much wider sand-body dimensions
than that for the channel alone, as observed in studies of modern rivers. This controversy, and an alternative interpretation, are discussed further in Sect.
9.5.3.
Nanson and Croke (1992) indicated that cohesive
banks and low stream power are the key characteristics that lead to the development of anastomosed
channels. N.D. Smith et al. (1989) suggested that the
anastomosed fluvial pattern may be but a transitional style that develops during a long-term process
of channel avulsion within a meandering system.
However, this has yet to be confirmed by later work.
The reversion to a single, stable channel observed
by these authors may be a reflection of the stable,
cratonic location of the river they studied. Work
by other researchers in steadily-subsiding basins
(e.g., D.G. Smith 1986; Shuster and Steidtmann 1987;
Kirschbaum and McCabe 1992; Nadon 1994) suggests that the anastomosed pattern may be dynamically stable for significant periods of geological time.
2.4.4.4 Ep hemeral Rivers
Miall (1977) proposed using the flood deposits of
Bijou Creek, C. olorado, as the type example of ·an
ephemeral stream, based On the work of McKee et al.
(1967). Subsequently, it has been shown that some of
the details of this model need modification, notably
the presumed predominance of the upper flow-regime plane-bed lithofacies (flat-bedded sandstone)
has been shown to be incorrect (F.G. Ethridge, pers.
comm., 1985).
Important new details of facies associations and
bed geometries in ephemeral streams were presented by Tunbridge (1981, 1984), Stear (1983), and
Wells (1983), based on well·exposed ancient examples. Tunbridge (1984) and Stear (1983) showed
that point-bar deposits, with lateral-accretion crossbedding, could develop into ephemeral streams.
Stear (1983) illustrated channel sandstones showing
Historical Background
both the ribbon and sheet geometries of Friend
(1983).
Deposits of some recent floods have been described by Lucchitta and Suneson (1981), Sneh
(1983), and Stear (1985), and include useful data on
sediment characteristics. Parkash et al. (1983) described the sedimentology of a modern low·gradient
'(terminal alluvial fan," on which flow decreases
downstream to zero at the distal edges, due to evaporation and infiltration. Some ancient deposits that
were possibly formed in this type of environment
were described by Friend (1978).
Detailed facies and architectural work has now
led to the recognition of several subtly different fluvial styles that develop under arid, ephemeral conditions. These are described at length in Chap. 8.
2.4.4.5 Large Rivers
Few thorough sedimentological studies of any
modern giant river have been undertaken since the
seminal contribution by Coleman (1969) on the
Brahmaputra. Bristow (1987, 1993a) reexamined this
river, as discussed in Sect. 2.4.4.2 (and in more detail
in Sect. 6.6).
Recognition of the deposits of giant rivers (including delta distributaries) requires identification
of bar complexes or channel scour surfaces with
appropriately large relief. A few recent studies have
achieved this. jones and McCabe (1980) described
complex cross-bed sets 40 m thick (Fig. 2.27) which
they attributed to deposition on alternate bars in a
deep deltaic distributary channel. Several studies of
the Athabasca Oil Sands, Alberta (Mossop and Flach
1983; Flach and Mossop, 1985) documented point
bars 25 m thick, also interpreted as the deposits of
distributary channels, probably in a tidally influenced environment. These deposits are discussed
fruther in Sect. 8.2.9. The deep fluvial channels in the
Morrison Formation, which were reconstructed by
Campbell (1976), have now been shown to be a result
of misinterpretation of the outcrop record (Cowan
1991; see Sects. 2.4.3.2, 10.3.2).
2.4.4.6 Floodplain Environments
The predominantly fine-grained deposits of the
floodplain have generally received much less detailed study than the coarse "framework, gravel and
sand units of the channels. Several commmentators
noted this at the first fluvial symposium. The major
appropriate, being small rivers in Banff National
Park, in narrow mountain valleys undergoing degradation, quite different from the large channels
within the broad, aggrading alluvial plain of the
Alberta foreland basin (as revealed by sandstone
body geometries). Putnam (1993) has noted one of
the major difficulties in relating surface facies studies to subsui'face interpretations) and that is that
"sand" bodies interpreted from wireline logs may
include crevasse and levee deposits, which, combined with the width of the actual channel deposit,
would indicate much wider sand-body dimensions
than that for the channel alone, as observed in studies of modern rivers. This controversy, and an alternative interpretation, are discussed further in Sect.
9.5.3.
Nanson and Croke (1992) indicated that cohesive
banks and low stream power are the key characteristics that lead to the development of anastomosed
channels. N.D. Smith et al. (1989) suggested that the
anastomosed fluvial pattern may be but a transitional style that develops during a long-term process
of channel avulsion within a meandering system.
However, this has yet to be confirmed by later work.
The reversion to a single, stable channel observed
by these authors may be a reflection of the stable,
cratonic location of the river they studied. Work
by other researchers in steadily-subsiding basins
(e.g., D.G. Smith 1986; Shuster and Steidtmann 1987;
Kirschbaum and McCabe 1992; Nadon 1994) suggests that the anastomosed pattern may be dynamically stable for significant periods of geological time.
2.4.4.4 Ep hemeral Rivers
Miall (1977) proposed using the flood deposits of
Bijou Creek, C. olorado, as the type example of ·an
ephemeral stream, based On the work of McKee et al.
(1967). Subsequently, it has been shown that some of
the details of this model need modification, notably
the presumed predominance of the upper flow-regime plane-bed lithofacies (flat-bedded sandstone)
has been shown to be incorrect (F.G. Ethridge, pers.
comm., 1985).
Important new details of facies associations and
bed geometries in ephemeral streams were presented by Tunbridge (1981, 1984), Stear (1983), and
Wells (1983), based on well·exposed ancient examples. Tunbridge (1984) and Stear (1983) showed
that point-bar deposits, with lateral-accretion crossbedding, could develop into ephemeral streams.
Stear (1983) illustrated channel sandstones showing
Historical Background
both the ribbon and sheet geometries of Friend
(1983).
Deposits of some recent floods have been described by Lucchitta and Suneson (1981), Sneh
(1983), and Stear (1985), and include useful data on
sediment characteristics. Parkash et al. (1983) described the sedimentology of a modern low·gradient
'(terminal alluvial fan," on which flow decreases
downstream to zero at the distal edges, due to evaporation and infiltration. Some ancient deposits that
were possibly formed in this type of environment
were described by Friend (1978).
Detailed facies and architectural work has now
led to the recognition of several subtly different fluvial styles that develop under arid, ephemeral conditions. These are described at length in Chap. 8.
2.4.4.5 Large Rivers
Few thorough sedimentological studies of any
modern giant river have been undertaken since the
seminal contribution by Coleman (1969) on the
Brahmaputra. Bristow (1987, 1993a) reexamined this
river, as discussed in Sect. 2.4.4.2 (and in more detail
in Sect. 6.6).
Recognition of the deposits of giant rivers (including delta distributaries) requires identification
of bar complexes or channel scour surfaces with
appropriately large relief. A few recent studies have
achieved this. jones and McCabe (1980) described
complex cross-bed sets 40 m thick (Fig. 2.27) which
they attributed to deposition on alternate bars in a
deep deltaic distributary channel. Several studies of
the Athabasca Oil Sands, Alberta (Mossop and Flach
1983; Flach and Mossop, 1985) documented point
bars 25 m thick, also interpreted as the deposits of
distributary channels, probably in a tidally influenced environment. These deposits are discussed
fruther in Sect. 8.2.9. The deep fluvial channels in the
Morrison Formation, which were reconstructed by
Campbell (1976), have now been shown to be a result
of misinterpretation of the outcrop record (Cowan
1991; see Sects. 2.4.3.2, 10.3.2).
2.4.4.6 Floodplain Environments
The predominantly fine-grained deposits of the
floodplain have generally received much less detailed study than the coarse "framework, gravel and
sand units of the channels. Several commmentators
noted this at the first fluvial symposium. The major