Facies Models
229
Fig. 8.43. Architectural model of a fine-grained meandering stream. Model 7 of Miall (1985)
deposits from sa nd-dominated, trough cross-bedded units to muddy IHS successions, and suggested
that these variations reflected varying transport energies within a meander bend (sand-dominated in
the upper part of the bend, mud dominated downstream), much as have been reported from coarsegrained meander bends. Wood argued that the
processes of meander evolution, including the
changes in local sediment load and slope caused by
neck and chute cutoffs, would affect the grain-size
and facies assemblage of the point-bar, accounting
for lateral variations in point-bar style within the
same macroform. Wood (1989) also reported the
presence of ribbon sandstones within a dominantly
sheet-sandstone succession, a-nd suggested that
these represented rapid cut-and-fill of channels, followed by avulsive abandonment before significant
lateral migration could occur.
8.2.1 0 Anastomosed River
The distinctiveness of the anastomosed fluvial style
was established by D .G. Smith and N.D. Smith (1980)
andD.G. Smith (1983), based on work on small modern rivers in the Xocky Mountains of Canada (Sect.
2.4.4.3). Most of the examples of this type of river
that have been described in the literature have low
gradients, and low stream power. Lateral channelmigration is minimal, and the floodplains therefore
lack scroll bars and oxbow lakes (Fig. 8.49). Channels
tend to be isolated - bounded in most places by
floodplain deposits. As these are normally finegrained, channel banks are typically cohesive and
steep-sided. Channel evolution takes the form of
crevassing, and the development of stable crevasse
channels, which may rejoin a main channel downstream and divert the flow from the channel from
which they branched. These characteristic& of anastomosed rivers apply in both humid and arid climates, in both settings anastomosed rivers are to be
found. Floodplains are characterized by muds which
may contain pond deposits, coals, calcretes, or
evaporites, depending on climate (Sect. 12.7). Sandy
crevasse-channel and splay deposits are also common. Table 4.4 lists the architectural elements identified in a typical ancient anastomosed fluvial
deposit, the Cutler Group ofNew Mexico. Figure 8.8J
is a typical vertical profile.
Channel bodies are typically straight to sinuous
ribbon sandstones (e.g., Fig. 6.5). Friend (1983) suggested that a width/depth ratio of < 15 is typical of
ribbon sandstones, but the new data and review
provided by Nadon (1994) indicate that a W/D ratio
of< 30 is a more appropriate definition. Commonly,
the channel sandstones consist of simple, cross-bedded channel elements, as in most of the channels
described by Eberth and Miall (1991). Figures 8.50
and 8.51 illustrate typical examples from that study.
Note the presence of minor internal erosion surfaces
in these sandstone bodies, These are third-order
surfaces that divide the sandstone bodies into depositional increments. In body B (Fig. 8.51), the inclination of the surfaces suggests local lateral accretion
and minor lateral channel migration. Kirschbaum
and McCabe (1992) demonstrated that anastomosed
channels may fill by lateral or vertical accretion or by
concentric accretion, in which bedding in the channel-fill parallels the channel floor. Nadon (1994)
demonstrated that in his examples the channel sandstones aggraded vertically at the same time as the
levees, so that channel and levee facies are
interbedded.
Crevasse channels and splays typically form complex sheet-sandstone bodies cut by numerous internal erosion surfaces. Figure 8.52 illustrates two large,
229
Fig. 8.43. Architectural model of a fine-grained meandering stream. Model 7 of Miall (1985)
deposits from sa nd-dominated, trough cross-bedded units to muddy IHS successions, and suggested
that these variations reflected varying transport energies within a meander bend (sand-dominated in
the upper part of the bend, mud dominated downstream), much as have been reported from coarsegrained meander bends. Wood argued that the
processes of meander evolution, including the
changes in local sediment load and slope caused by
neck and chute cutoffs, would affect the grain-size
and facies assemblage of the point-bar, accounting
for lateral variations in point-bar style within the
same macroform. Wood (1989) also reported the
presence of ribbon sandstones within a dominantly
sheet-sandstone succession, a-nd suggested that
these represented rapid cut-and-fill of channels, followed by avulsive abandonment before significant
lateral migration could occur.
8.2.1 0 Anastomosed River
The distinctiveness of the anastomosed fluvial style
was established by D .G. Smith and N.D. Smith (1980)
andD.G. Smith (1983), based on work on small modern rivers in the Xocky Mountains of Canada (Sect.
2.4.4.3). Most of the examples of this type of river
that have been described in the literature have low
gradients, and low stream power. Lateral channelmigration is minimal, and the floodplains therefore
lack scroll bars and oxbow lakes (Fig. 8.49). Channels
tend to be isolated - bounded in most places by
floodplain deposits. As these are normally finegrained, channel banks are typically cohesive and
steep-sided. Channel evolution takes the form of
crevassing, and the development of stable crevasse
channels, which may rejoin a main channel downstream and divert the flow from the channel from
which they branched. These characteristic& of anastomosed rivers apply in both humid and arid climates, in both settings anastomosed rivers are to be
found. Floodplains are characterized by muds which
may contain pond deposits, coals, calcretes, or
evaporites, depending on climate (Sect. 12.7). Sandy
crevasse-channel and splay deposits are also common. Table 4.4 lists the architectural elements identified in a typical ancient anastomosed fluvial
deposit, the Cutler Group ofNew Mexico. Figure 8.8J
is a typical vertical profile.
Channel bodies are typically straight to sinuous
ribbon sandstones (e.g., Fig. 6.5). Friend (1983) suggested that a width/depth ratio of < 15 is typical of
ribbon sandstones, but the new data and review
provided by Nadon (1994) indicate that a W/D ratio
of< 30 is a more appropriate definition. Commonly,
the channel sandstones consist of simple, cross-bedded channel elements, as in most of the channels
described by Eberth and Miall (1991). Figures 8.50
and 8.51 illustrate typical examples from that study.
Note the presence of minor internal erosion surfaces
in these sandstone bodies, These are third-order
surfaces that divide the sandstone bodies into depositional increments. In body B (Fig. 8.51), the inclination of the surfaces suggests local lateral accretion
and minor lateral channel migration. Kirschbaum
and McCabe (1992) demonstrated that anastomosed
channels may fill by lateral or vertical accretion or by
concentric accretion, in which bedding in the channel-fill parallels the channel floor. Nadon (1994)
demonstrated that in his examples the channel sandstones aggraded vertically at the same time as the
levees, so that channel and levee facies are
interbedded.
Crevasse channels and splays typically form complex sheet-sandstone bodies cut by numerous internal erosion surfaces. Figure 8.52 illustrates two large,